[PATCH] i386: more vsyscall documentation

Document a limitation of vsyscall-sysenter, since patches to fix it have
been rejected.

Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>