[llvm] [Docs][Github] explain how to rectify gh pr merge failure (PR #66223)

[Nick Desaulniers via llvm-commits]

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@@ -131,11 +131,45 @@ You can also merge via the CLI by switching to your branch locally and run:
 
   gh pr merge --squash --delete-branch
 
+If you observe an error message from the above informing you that your pull
+request is not mergeable, then that is likely because upstream has been
+modified since your pull request was authored in a way that now results in a
+merge conflict. You must first resolve this merge conflict in order to merge
+your pull request. In order to do that:
+
+::
+
+  git fetch origin
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nickdesaulniers wrote:

done in 5f8e1064311a

https://github.com/llvm/llvm-project/pull/66223