Add missing documentation

[fmigneault]

Following magpie features have no explicit mention or setup details. Relevant sections should be created so that they can be found by new users.

• resource auto-sync feature for a given service and cron configuration setup for it (fixed by 29b907f)
• listing and explanation/features of every available service types (fixed by 29b907f)
• explicit description of user/group vs service/resource objects and their relationships
• description of flags that modify request results (effective, inherited, cascade, list) (see descriptions in [Feature] Effective permission API responses #341)
• distinction between different kind of permissions:
  • applied permissions : entry for a given service/resource-user/group combination
  • effective permissions: resolution of applied permissions of user & his group membership
  • access permissions: allowed level to routes to request details (ie: roles) (wip Milestone 2.0 - User scope views, Discoverable Groups & Patches #340)
  • ownership permissions: extra set of permissions to user/group that owns the service/resource (?)
    (provides access permissions to some specific routes matching the service/resource & provide all applied permissions for it)
• distinction of applied permission modifiers ([Feature] Improve user/group permissions scheme #342):
  • match vs recur
    • recur: recursive to this and all children resources: (implicit for backward compat when no modifier provided)
    • match: only applied onto this specific resource
  • allow vs deny (Support of DENY access #235)
    • allow: default when permission is added (for backward compat)
    • deny: explicit override of any previous 'allow' (including recursive of parent resource, no permission equals deny)
• AuthN details refer to specific issue ([Feature] Document authentication methods and related authenticated-user details #344)