x86/headers: Clean up and better document uapi/asm/sigcontext.h

Clean up sigcontext.h:

 - the explanations were full of typos and were hard to read in general
 - use consistent and readable vertical spacing
 - fix, harmonize and extend comments

No field name has been changed, user-space might be relying on
them.
Acked-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1441438363-9999-4-git-send-email-mingo@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>