x86, doc: Clarify the use of asm("%edx") in uaccess.h

Put in a comment that explains that the use of asm("%edx") in
uaccess.h doesn't actually necessarily mean %edx alone.

Cc: Jamie Lokier <jamie@shareable.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. J. Lu <hjl.tools@gmail.com>
Link: http://lkml.kernel.org/r/511ACDFB.1050707@zytor.com


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>