vinyl: update comments, remove duplciate comments
vinyl: update comments, remove duplciate comments

Keep comments in one place. When we move the forward declarations
to a header file, the comments will migrate along.  For now, keep
them near function definitions: they'll get improvements, and it'd
be difficult to maintain two copies otherwise.

mejedi/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool,rtsisyk/tarantool,mejedi/tarantool