objtool: Update documentation

The objtool documentation is very stack validation centric.  Broaden the
documentation and describe all the features objtool supports.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Link: https://lkml.kernel.org/r/b6a84d301d9f73ec6725752654097f4e31fa1b69.1650300597.git.jpoimboe@redhat.com