objtool: Fix up some outdated references to ENTRY/ENDPROC

ENTRY and ENDPROC were deprecated years ago and replaced with
SYM_FUNC_{START,END}.  Fix up a few outdated references in the objtool
documentation and comments.  Also fix a few typos.

Suggested-by: Brendan Jackman <jackmanb@google.com>
Suggested-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/5eb7e06e1a0e87aaeda8d583ab060e7638a6ea8e.1742852846.git.jpoimboe@kernel.org