Cleanup enum values, allow using identifier+value as an alternative to @codegen_name

compiler/src/model/utils.ts

@@ -440,9 +440,14 @@ export function modelEnumDeclaration (declaration: EnumDeclaration): model.Enum
       .map(m => {
         // names that contains `.` or `-` will be wrapped inside single quotes
         const name = m.getName().replace(/'/g, '')
-        const member = {
+        const member: model.EnumMember = {
           name: name
         }
+        const value = m.getValue()
+        if (value && (typeof value === 'string')) {
+          member.name = value
+          member.codegenName = name
+        }
         hoistEnumMemberAnnotations(member, m.getJsDocs())
 
         return member

docs/modeling-guide.md

@@ -75,18 +75,45 @@ property: Dictionary<string, string | long>
 
 ### Enum
 
-Represents a set of allowed values:
+Represents a set of allowed values.
+
 
 ```ts
 enum MyEnum {
-  first = 0,
-  second = 1,
-  third = 2
+  first,
+  second,
+  third
 }
 
 property: MyEnum
 ```
 
+Note that you don't have to provide both identifiers and values as is common in TypeScript. When there's only an identifier, the API specification compiler will use it for both the identifier and the value of enum members.
+
+Also do not use identifiers and values for the sole purpose of providing upper-case identifiers (e.g. `FOO = 'foo'`). Each language generator will use the identifier casing that is expected by that language.
+
+Some enumerations contain values that aren't identifiers, or that are not explicit enough. In that case you can either assign values to enum members or use the `@codegen_name` jsdoc tag to define the identifier that will be used by code generators:
+
+```ts
+enum MyEnum {
+    percent_of_sum,
+    mean,
+    /** @codegen_name z_score */
+    'z-score',
+    softmax
+}
+
+export enum IntervalUnit {
+    second = 's',
+    minute = 'm',
+    hour = 'h',
+    day = 'd',
+    week = 'w'
+}
+```
+
+**Use custom identifiers for enum members sparingly**: we want to keep identifiers as close as possible to their value in JSON payloads to that usesr can easily map values found in the Elasticsearch reference documentation to code identifiers in the client libraries. 
+
 Some enumerations accept alternate values for some of their members. The `@aliases` jsdoc tac can be used to capture these values:
 
 ```ts