[llvm][docs][nfc] Add a note on GitHub's "squash and merge"

From what I can tell, many people are still unaware that in LLVM we use
the PR summary (rather than PR commit messages) as the message for the
final commit to be merged. This is particularly unclear to folks without
commit access (folks with commit access can edit the commit message just
before merging).

Author: banach-space

llvm/docs/GitHub.rst

@@ -214,6 +214,13 @@ commonly used first:
   request will understand that you're rebasing just your patches, and display
   this result correctly with a note that a force push did occur.
 
+.. note::
+   When using `Squash and merge` in LLVM’s GitHub web interface, GitHub will
+   use the pull request title and description as the commit message. Only
+   users with commit access can edit the commit message just before merging a PR.
+
+   If you do not have write permissions, make sure the PR title and description
+   are **up to date** before the review process concludes.
 
 Pre-merge Continuous Integration (CI)
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