Documentation: seqlock: s/LOCKTYPE/LOCKNAME/g

Sequence counters with an associated write serialization lock are called
seqcount_LOCKNAME_t. Fix the documentation accordingly.

While at it, remove a paragraph that inappropriately discussed a
seqlock.h implementation detail.

Fixes: 6dd699b1 ("seqlock: seqcount_LOCKNAME_t: Standardize naming convention")
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20201206162143.14387-2-a.darwish@linutronix.de