Better explain tracker resolution causes and fix versions

This is part of https://tracker.moodle.org/browse/MDLSITE-7354

In that issue, it has been decided to start applying some
simple rules to better control resolution causes and fix versions
in the MDL project.

These changes just insert that information within the Tracker guide
(note that some of the points already were present)

general/development/tracker/guide.md

@@ -52,7 +52,7 @@ Once an issue has been created, the following additional fields are able to be c
 
 | Field  | Values  | Notes  |
 |---|---|---|
-| **Fixed Version/s**  | <ul><li>Prior to integration, this will be set to a backlog (a queue of development work), for example `Must fix for X`.</li><li>After integration, this will be set to the Moodle version the bug was fixed in, for example `4.0.1`.</li></ul>  | <ul><li>This is usually set by an integrator.</li><li>Not to be confused with `Affected version`, which is used to define the Moodle version where the issue can be reproduced.</li><li>If you resolve the bug as anything but `Fixed` (`Cannot Reproduce`, `Won't Fix`, etc.) leave **Fix Version/s** blank.</li><li>**Fix version/s** are used to automatically build release notes (see the tabs on <http://tracker.moodle.org/browse/MDL>)</li></ul>  |
+| **Fixed Version/s**  | <ul><li>Prior to integration, this will be blank or set to a backlog (a queue of development work), for example `Must fix for X`.</li><li>After integration, this will be set to the Moodle version(s) the issue was fixed in, for example `4.0.1`.</li><li>For more detailed information, look to the **Resolution** field bellow.</li></ul>  | <ul><li>This is usually set by an integrator.</li><li>Not to be confused with `Affected version`, which is used to define the Moodle version where the issue can be reproduced.</li><li>If you resolve the bug as anything but `Fixed` and, sometimes, `Done` (like `Cannot Reproduce`, `Won't Fix`, etc.) leave **Fix Version/s** blank.</li><li>**Fix version/s** are used to automatically build release notes (see the tabs on <http://tracker.moodle.org/browse/MDL>)</li></ul>  |
 | **Priority**  | <ul><li>**Blocker**<br/>Blocks development or testing, prevents Moodle from running. Applicable to bugs only.</li><li>**Critical**<br/>Crashes server, loss of data, severe memory leak</li><li>**Major**<br/>Major loss of function, incorrect output</li><li>**Minor**<br/>Minor loss of function where workaround is possible</li><li>**Trivial**<br/>Cosmetic problem like misspelt words or misaligned text</li></ul>  | <ul><li>When it is reported, the priority level represents the severity of a bug.</li><li>After being reported, the priority may be promoted by HQ developers and component leads as an issue escalates.</li><li>Other users wishing to influence the priority of issues should do so by voting for the issue.</li><li>The priority of new features and improvements should generally remain at the default (Minor) level.</li></ul>  |
 | **Reporter**  | The person who logs the bug.<br/>This field is automatically filled by Tracker.  |   |
 | **Assignee**  | The person who will fix the issue. The assignee should be set when there is a definite intention to complete the issue.  | <ul><li>Developers or QA Testers can reassign issues.</li><li>Please note that even though a person may be assigned to an issue, this does not mean they are currently working on the issue, although they are likely to in future.</li></ul>  |
@@ -76,7 +76,7 @@ Once an issue has been created, the following additional fields are able to be c
 
 | Field  | Values  | Notes  |
 |---|---|---|
-| **Resolution**  | <ul><li>**Fixed**<br/>Bug has been fixed; a code change has been integrated into Moodle code.</li><li>**Won't Fix**<br/>The problem described is an issue which will never be fixed. Specific reasons should be given.</li><li>**Not a bug**<br/>This issue is not a bug. The issue may have been logged in error. Use this code if the bug was fixed by another bug report or in some earlier Moodle version.</li><li>**Duplicate**<br/>The problem is a duplicate of an existing issue</li><li>**Incomplete**<br/>More information was needed to understand this bug, but it was not provided.</li><li>**Can't Reproduce**<br/>Attempts at reproduce the issue failed. If more information appears later, please open a new issue.</li><li>**Deferred**<br/>The resolution to this bug will be deferred to a later release or to a fix in a third-party plugin used in Moodle.</li></ul>  | This field is only displayed when resolving or closing a bug.  |
+| **Resolution**  | Issues that may/must have the **Fixed versions** field filled:<ul><li>**Fixed**<br/>Issue has been fixed; a code change has been integrated into Moodle code. It's **mandatory** to set the *Fixed versions* field for these issues.</li><li>**Done**<br/>Normally used for tasks, epics... issues that don't "own" code changes in Moodle, but still have required actions (planning, review, adjust some related system...). When relevant or clearly related with any release it's **recommended** to set the *Fixed versions* to them.</li></ul>Issues that must not have the **Fixed versions** field filled:<ul><li>**Won't Fix**<br/>The problem described is an issue which will never be fixed. Specific reasons should be given.</li><li>**Not a bug**<br/>This issue is not a bug. The issue may have been logged in error. Use this code if the bug was fixed by another bug report or in some earlier Moodle version.</li><li>**Duplicate**<br/>The problem is a duplicate of an existing issue</li><li>**Incomplete**<br/>More information was needed to understand this bug, but it was not provided.</li><li>**Can't Reproduce**<br/>Attempts at reproduce the issue failed. If more information appears later, please open a new issue.</li><li>**Deferred**<br/>The resolution to this bug will be deferred to a later release or to a fix in a third-party plugin used in Moodle.</li></ul>  | This field is only displayed when resolving or closing a bug.  |
 
 ## See also