In connection to MDL-26342, this also extends help text for custom profile fields.
A description field already exists for each custom profile field. This is never displayed to users, except during the editing of the custom field.
The most trivial fix here is for the functions which control the display of custom fields on the profile and sign-up screens to add the description field information, in a suitably classed <div>. But this may cause problems for Moodle installations with content in that field already who don't expect it to be displayed.
I see two options: 1) add a new text field in addition to description which would be displayed on the sign-up and profile fields; 2) add a check box which would control whether the description text should be displayed or not on the sign-up and profile fields. Neither of these options would have impact for users upgrading existing custom profile fields. Option 2 requires less database storage and less inputting by users, so is my preferred solution.
I have a number of requests for this (for our Sept 2011 release) and am more than happy to program myself and offer a patch. If whoever triages this thinks that's OK, please assign to me and I will add to the OU development schedule over the next few months.