Document 'x-enum-descriptions' and 'x-enum-varnames' (OpenAPITools#2010)

Author: jmini

docs/templating.md

@@ -615,6 +615,41 @@ For example, suppose you use your specification document for code generation wit
 
 The following are vendor extensions supported by OpenAPI Generator. The list may not be up-to-date, the best way is to look for "x-" in the built-in mustache templates.
 
+### All generators (core)
+
+#### Enum
+
+`x-enum-varnames` can be used to have an other enum name for the corresponding value.
+This is used to define names of the enum items.
+
+`x-enum-descriptions` can be used to provide an individual description for each value.
+This is used for comments in the code (like javadoc if the target language is java).
+
+`x-enum-descriptions` and `x-enum-varnames` are each expected to be list of items containing the same number of items as `enum`.
+The order of the items in the list matters: their position is used to group them together.
+
+Example:
+
+```yaml
+WeatherType:
+  type: integer
+  format: int32
+  enum:
+    - 42
+    - 18
+    - 56
+  x-enum-descriptions:
+    - 'Blue sky'
+    - 'Slightly overcast'
+    - 'Take an umbrella with you'
+  x-enum-varnames:
+    - Sunny
+    - Cloudy
+    - Rainy
+```
+
+In the example for the integer value `42`, the description will be `Blue sky` and the name of the enum item will be `Sunny` (some generators changes it to `SUNNY` to respect some coding convention).
+
 ### ObjC
 #### x-objc-operationId