docs(lfs): document the sha the contents API expects for LFS files

docs/administration/git-lfs-support.md

@@ -41,3 +41,24 @@ client that causes SSH transfers to hang: https://github.com/git-lfs/git-lfs/pul
 This can be worked around on all the client machines by setting the git config:
 `git config --global lfs.ssh.automultiplex false`
 :::
+
+# Changing LFS files through the API
+
+The [contents API](https://docs.gitea.com/api/operations/repo-change-files/)
+works on LFS tracked files as well: send the file content as usual and Gitea
+stores it as an LFS object and commits a pointer file, as long as the path is
+matched by a `filter=lfs` rule in `.gitattributes`.
+
+What is easy to get wrong is the `sha` of the file being changed, which
+`PUT`/`DELETE /repos/{owner}/{repo}/contents/{filepath}` and
+`POST /repos/{owner}/{repo}/contents` require for an existing file. Gitea
+compares it with the ID of the blob the commit has at that path, and for an LFS
+tracked file that blob is the **pointer file**, not the content:
+
+- use the `sha` returned by
+  [`GET /repos/{owner}/{repo}/contents/{filepath}`](https://docs.gitea.com/api/operations/repo-get-contents/)
+- do not use `lfs_oid` from the same response, which identifies the LFS object,
+  and do not use a checksum of the file itself
+
+Sending anything else fails with `sha does not match`, even though the request
+looks correct.