Add notes about SHAs in Diff APIs

doc/api/merge_requests.md

@@ -761,6 +761,8 @@ the `approvals_before_merge` parameter:
 }
 ```
 
+The `diff_refs` in the response correspond to the latest diff version of the merge request.
+
 ## Get single MR participants
 
 Get a list of merge request participants.
@@ -2328,7 +2330,8 @@ Example response:
 
 ## Get MR diff versions
 
-Get a list of merge request diff versions.
+Get a list of merge request diff versions. For an explanation of the SHAs in the response,
+read [SHAs in the API response](#shas-in-the-api-response).
 
 ```plaintext
 GET /projects/:id/merge_requests/:merge_request_iid/versions
@@ -2367,9 +2370,16 @@ Example response:
 }]
 ```
 
+### SHAs in the API response
+
+- `head_commit_sha`: The HEAD commit of the source branch.
+- `base_commit_sha`: The merge-base commit SHA between the source branch and the target branches.
+- `start_commit_sha`: The HEAD commit SHA of the target branch when this version of the diff was created.
+
 ## Get a single MR diff version
 
-Get a single merge request diff version.
+Get a single merge request diff version. For an explanation of the SHAs in the response,
+read [SHAs in the API response](#shas-in-the-api-response).
 
 ```plaintext
 GET /projects/:id/merge_requests/:merge_request_iid/versions/:version_id