docs: cal: add --color option description to manual page

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

misc-utils/cal.1

@@ -65,6 +65,20 @@ Display Julian dates (days one-based, numbered from January 1).
 \fB\-y\fR, \fB\-\-year\fR
 Display a calendar for the current year.
 .TP
+\fB\-\-color\fR [\fIwhen\fR]
+Colorize output.  The
+.I when
+can be
+.IR never ,
+.IR auto ,
+or
+.IR always .
+Never will turn off colorizing in all situations.  Auto is default, and
+it will make colorizing to be in use if output is done to terminal.
+Always will allow colors to be outputed when
+.B cal
+outputs to pipe, or is called from a script.
+.TP
 \fB\-V\fR, \fB\-\-version\fR
 Display version information and exit.
 .TP