textsearch: fix kernel-doc warnings and add kernel-api section

Make lib/textsearch.c usable as kernel-doc.
Add textsearch() function family to kernel-api documentation.
Fix kernel-doc warnings in <linux/textsearch.h>:
  ../include/linux/textsearch.h:65: warning: Incorrect use of kernel-doc format:
	* get_next_block - fetch next block of data
  ../include/linux/textsearch.h:82: warning: Incorrect use of kernel-doc format:
	* finish - finalize/clean a series of get_next_block() calls

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>