Docs: relayd: received metadata position is reset on clear

Correct a comment in the relayd documentation that incorrectly mentioned
the 'sent' position being reset by the 'clear' command.

The correct behavior resets the metadata stream's 'received' position to
'0', not the 'sent' position. The relay daemon expects to re-receive the
metadata contents that matches the previous contents up to the previous
'received' position.

The client, however, does not expect to receive the original contents of
the metadata stream a second time.

Note that from the relay daemon's perspective, a "clear" command does
not exist per se. It is implemented as a stream rotation that moves the
streams from a trace chunk that has an associated 'DELETE' close command
to a new one (which may also be a 'nil' chunk).

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I598fe736c57ab3e934ff0207674d0ecff2bf3e74