main: add -q/-Q options to set clock/print offset once and exit

chrony.texi.in

@@ -1026,6 +1026,12 @@ system clock last time the computer was on.
 @item -u <user>
 When this option is used, chronyd will drop root privileges to the specified
 user.  So far, it works only on Linux when compiled with capabilities support.
+@item -q
+When run in this mode, @code{chronyd} will set the system clock once
+and exit.  It will not detach from the terminal.
+@item -Q
+This option is similar to `-q', but it will only print the offset and
+not correct the clock.
 @item -v
 This option displays @code{chronyd's} version number to the terminal and
 exits.