Add comments.

app/services/files/update_service.rb

@@ -33,7 +33,7 @@ module Files
       if created_successfully
         success
       else
-        error("Your changes could not be committed. Maybe the file changed, or there was nothing to commit?")
+        error("Your changes could not be committed. Maybe the file was changed by another process or there was nothing to commit?")
       end
     end
   end

doc/api/repository_files.md

@@ -87,6 +87,8 @@ error message. Possible causes for a failed commit include:
   user tried to make an empty commit;
 - the branch was updated by a Git push while the file edit was in progress.
 
+Currently gitlab-shell has a boolean return code, preventing GitLab from specifying the error.
+
 ## Delete existing file in repository
 
 ```