api: api_info()['version']

API level is disconnected from NVIM version. The API metadata holds the
current API level, and the lowest backwards-compatible level supported
by this instance.

Release 0.1.6 is the first release that reports the Nvim version and API
level.

    metadata['version'] = {
      major: 0,
      minor: 1,
      patch: 6,
      api_level: 1,
      api_compatible: 0,
      api_prerelease: false,
    }

The API level may remain unchanged across Nvim releases if the API has
not changed.

When changing the API,
    - set NVIM_API_PRERELEASE to true
    - increment NVIM_API_LEVEL (at most once per Nvim version)
    - adjust NVIM_API_LEVEL_COMPAT if backwards-compatibility was broken

api_level_0.mpack was generated from Nvim 0.1.5 with:
    nvim --api-info

1 files changed, 2 insertions, 3 deletions

diff --git a/runtime/doc/msgpack_rpc.txt b/runtime/doc/msgpack_rpc.txt
index bfd4100f15..757f5574d4 100644
--- a/runtime/doc/msgpack_rpc.txt
+++ b/runtime/doc/msgpack_rpc.txt
@@ -168,9 +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 "api_level" key contais API compatibility information. The "current"
-    key holds the API version supported Neovim. The "compatibility" key holds
-    the oldest supported API version.
+  - 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