x86/bugs: Add RSB mitigation document

Create a document to summarize hard-earned knowledge about RSB-related
mitigations, with references, and replace the overly verbose yet
incomplete comments with a reference to the document.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/ab73f4659ba697a974759f07befd41ae605e33dd.1744148254.git.jpoimboe@kernel.org