x86/mm: Improve documentation for low-level device I/O functions

Add kerneldoc comments for memcpy_{to,from}io() and memset_io().  The
existing documentation for ioremap() was distant from the definition,
causing kernel-doc to miss it; move it appropriately.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170127161752.0b95e95b@lwn.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>