X.man: move XLOCAL documentation from Xorg(1) man page

These settings affect clients, not server, so belong here, next to
the information about how to set $DISPLAY.

Also updated to drop ISC and correct description of Unix domain sockets.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>