Update commits.md to add introduction

doc/api/commits.md

@@ -7,6 +7,8 @@ type: reference, api
 
 # Commits API **(FREE)**
 
+This API operates on [repository commits](https://git-scm.com/book/en/v2/Git-Basics-Recording-Changes-to-the-Repository). Read more about [GitLab-specific information](../user/project/repository/index.md#commits) for commits.
+
 ## List repository commits
 
 Get a list of repository commits in a project.