server: Add doxygen comment for wl_client_for_each

Commit 596024f7 includes a doc comment
with a link to the wl_client_for_each macro, causing an error when generating
documentation.

Add a doc comment to wl_client_for_each, enabling the hyperlink and removing
the error.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>