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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

>Maybe consider merging this as discussed in [#93562 (comment)](https://github.com/llvm/llvm-project/pull/93562#issuecomment-2140180221)...?

Yes, that should happen soon - before doing so it had just occurred to me that this guide assumes a unix shell, but it might be appropriate to add some windows equivalents, which I wanted to verify before adding and just haven't gotten to yet. Though if you'd prefer that to go in a separate review afterwards, I can just merge this now.

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