Merge #5535 from justinmk/api_level

api: Nvim version + API level

runtime/doc/api.txt

@@ -51,9 +51,10 @@ Tabpage				  -> enum value kObjectTypeTabpage
 
 Nvim exposes metadata about the API as a Dictionary with the following keys:
 
-functions  	calling signature of the API functions
-types		The custom handle types defined by Nvim
-error_types	The possible kinds of errors an API function can exit with.
+version		Nvim version, API level/compatibility
+functions  	API function signatures
+types		Custom handle types defined by Nvim
+error_types	Possible error types returned by API functions
 
 This metadata is mostly useful for external programs accessing the API via
 RPC, see |rpc-api|.

runtime/doc/msgpack_rpc.txt

@@ -168,6 +168,8 @@ API metadata object ~
 API clients exist to hide msgpack-rpc details. The API metadata object
 contains information that makes this task easier (see also |rpc-types|):
 
+  - The "version" key contains the Nvim version, API level, and API
+    backwards-compatibility level.
   - The "functions" key contains a list of metadata objects for individual
     functions.
   - Each function metadata object has |rpc-types| information about the return