?make_option should mention "callback" as a possible value for action

[greg-minshall]

if you provide a callback function, but don't specify action="callback", you get a warning.

and, the man page ?make_option (or ?add_option) doesn't mention "callback" as a possible value for the action parameter.

[trevorld]

if you provide a callback function, but don't specify action="callback", you get a warning.

This seems to match the behaviour of the API I'm mimicking. Not sure I want to change that.

and, the man page ?make_option (or ?add_option) doesn't mention "callback" as a possible value for the action parameter.

I should improve the man page callback documentation.