seqlock: Add kernel-doc for seqcount_t and seqlock_t APIs

seqlock.h is now included by kernel's RST documentation, but a small
number of the the exported seqlock.h functions are kernel-doc annotated.

Add kernel-doc for all seqlock.h exported APIs.
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200720155530.1173732-6-a.darwish@linutronix.de