Add documentation for reliable stacktrace. This is intended to describe
the semantics and to be an aid for implementing architecture support for
HAVE_RELIABLE_STACKTRACE.

Unwinding is a subtle area, and architectures vary greatly in both
implementation and the set of concerns that affect them, so I've tried
to avoid making this too specific to any given architecture. I've used
examples from both x86_64 and arm64 to explain corner cases in more
detail, but I've tried to keep the descriptions sufficient for those who
are unfamiliar with the particular architecture.

This document aims to give rationale for all the recommendations and
requirements, since that makes it easier to spot nearby issues, or when
a check happens to catch a few things at once.

Signed-off-by: Mark Rutland
[Updates following review -- broonie]
Acked-by: Josh Poimboeuf
Reviewed-by: Randy Dunlap
Signed-off-by: Mark Brown
Signed-off-by: Jiri Kosina