[RemoveDIs] Update migration doc to explain how to handle textual IR updates (#91725)

Related review: https://github.com/llvm/llvm-project/pull/91724

This patch updates the RemoveDIs migration document to include details
on the textual IR changes, including steps to update any downstream lit
tests accordingly. These steps are the same as those used to update the
lit tests in the LLVM/Clang lit tests, as detailed in the review linked
above.