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

[J. Ryan Stinnett via llvm-commits]

jryans wrote:

> 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.

Hmm, I guess I'd say it's better to have migration docs available sooner rather than later for anyone who may need them. Not sure how many LLVM developers are on Windows to help prioritise those additions... Maybe aim to merge this sometime this week, and if the Windows bits aren't ready this week, then add them in separate PR...?

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