[ReleaseNotes][RemoveDIs] Add release note for deprecated insertion methods

Author: jmorse

llvm/docs/ReleaseNotes.md

@@ -116,6 +116,8 @@ Changes to the LLVM IR
 Changes to LLVM infrastructure
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+ * Several methods that use Instruction pointers as insertion positions (moveBefore, getFirstNonPHI) have been deprecated in favour of overloads and variants that use `BasicBlock::iterator`s instead. The instruction-flavoured methods will be removed in a future release. This work is part of the [RemoveDIs](https://llvm.org/docs/RemoveDIsDebugInfo.html) project, the documentation for which contains instructions for updating call-sites using the deprecated methods.
+
 Changes to building LLVM
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