Documentation: seq_file: Document seq_open_private(), seq_release_private()

Despite the fact that these functions have been around for years, they
are little used (only 15 uses in 13 files at the preseht time) even
though many other files use work-arounds to achieve the same result.

By documenting them, hopefully they will become more widely used.

Signed-off-by: Rob Jones <rob.jones@codethink.co.uk>
Acked-by: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>