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

[Anatoly Trosinenko via llvm-commits]

https://github.com/atrosinenko commented:

@hvdijk Thank you for your review! Considering two `FIXME`s in the documentation itself - they were intended to be publicly discussed and resolved before merging this PR. When writing an explanation why we may want LR to be trusted as opposed to just safe-to-dereference before tail call is performed, I found this quite obscure, so I wanted to ask for comments.

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