docbook: move pipe and splice to filesystems docbook

Move pipes and splice docbook to filesystems book.
kernel-api book is huge (10x most other books) & slow to process.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>