credential-cache--daemon: clarify "exit" action semantics

credential-cache--daemon: clarify "exit" action semantics

When this code was originally written, there wasn't much
thought given to the timing between a client asking for
"exit", the daemon signaling that the action is done (with
EOF), and the actual cleanup of the socket.

However, we need to care about this so that our test scripts
do not end up racy (e.g., by asking for an exit and checking
that the socket was cleaned up). The code that is already
there happens to behave very reasonably; let's add a comment
to make it clear that any changes should retain the same
behavior.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>