Modify notes about `duration` unit in commits and merge_requests API documentation [ci skip]

doc/api/commits.md

@@ -83,7 +83,7 @@ curl -H "PRIVATE_TOKEN: 9koXpg98eAheJpvBs5tK" "https://gitlab.example.com/api/v3
 }
 ```
-`duration` in `last_build` - value exposed in **seconds**
+**Note:** The `duration` value as exposed in `last_build`, is in seconds.
 ## Get the diff of a commit

doc/api/merge_requests.md

@@ -121,7 +121,7 @@ Parameters:
 }
 ```
-`duration` in `last_commit` > `last_build` - value exposed in **seconds**
+**Note:** The `duration` value as exposed in `last_commit > last_build`, is in seconds.
 ## Get single MR commits