add kernel-doc for simple_read_from_buffer and memory_read_from_buffer

Add kernel-doc comments describing simple_read_from_buffer and
memory_read_from_buffer.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: "Randy.Dunlap" <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>