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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

Rebased the docs to remove the existing conflict, and added some minor adjustments to the start of the "Textual IR" section, just cleaning up some of the terminology and unclear sentences.

https://github.com/llvm/llvm-project/pull/91725