ext4: Update stale comment about write constraints

The comment above do_journal_get_write_access() is very stale. Most of
it just does not refer to what the function does today or how jbd2
works. The bit about transaction handling during write(2) is still
correct so just update the function names in that part and move the
comment to a more appropriate place.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20230228051319.4085470-2-tytso@mit.edu