[llvm] Update the backporting docs (PR #100401)

[Louis Dionne via llvm-commits]

================
@@ -376,10 +376,10 @@ Releases
 
 Backporting Fixes to the Release Branches
 -----------------------------------------
-You can use special comments on issues to make backport requests for the
-release branches.  This is done by making a comment containing the following
-command on any issue that has been added to one of the "X.Y.Z Release"
-milestones.
+You can use special comments on issues or pull requestes to make backport
----------------
ldionne wrote:

```suggestion
You can use special comments on issues or pull requests to make backport
```

https://github.com/llvm/llvm-project/pull/100401