[llvm] [BOLT] Rework user-facing documentation of BOLT gadget scanner (PR #176446)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

Just wanted to say that I'm happy with dropping the fixmes and migrating them to LLVM issues, so that this much improved documentation can land. Since it seems these are mostly reactions/updates to @hvdijk's feedback, I'll let @hvdijk resolve the conversation/accept the PR is he's available.

https://github.com/llvm/llvm-project/pull/176446