eventloop: clarify post_dispatch_check()

This *technically* changes the semantics of the return value of the source callbacks.
Previously you could return a negative number from a source callback and it would prevent
*other* source callbacks from triggering a subsequent recheck.

Doing that seems like such a bad idea it's not worth supporting.

v2: Log this case if it is hit, so we don't silently change behaviour.

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>