[Docs][llvm-exegesis] Refactor snippet annotations in documentation

Currently, the llvm-exegesis documentation page has all
snippet annotation information under an example. This patch refactors
the annotation documentation to a separate section to make things more
clear and to make adding future annotations easier. This patch also
significantly expands the documentation on the memory scratch space to
which a pointer can be passed through a register as the documentation on
this was quite sparse previously.

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D146890