Improve semaphore documentation

Move documentation from semaphore.h to semaphore.c as requested by
Andrew Morton.  Also reformat to kernel-doc style and add some more
notes about the implementation.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>

diff --cc include/linux/semaphore.h
Simple merge

diff --cc kernel/semaphore.c
Simple merge