Description in system directives; a summary for complex prompts

[gregoryg]

I add descriptions in my growing list of long complex gptel directives.

I find it helpful to use as a memory jog or summary of what long complex prompts are supposed to accomplish.

It has proven useful in a custom non-transient-mode function that uses annotations and looks like this:

Note that any prompt with no description set shows as "NO DESCRIPTION".

This change includes the definition needed for the customization framework, and still works with the existing transient selection framework.

[gregoryg]

please hold off on this - in current version I see that transient mode selection is not working - I will investigate

[gregoryg]

closing in order to do more testing with gptel-transient.el