For simple definitions, that would work.  However, we expose a good 
deal more information than could fit in a title attribute. For 
example, our issue tracking overview:

https://mail.ideacode.com/wicked/display.php?page=IssueTracking

shows that tables and images are vital information in our case.

Perhaps a middle of the road approach would work.  Provide a text 
input field and a select with two options: description and external 
link.  If you type text and select description, the text appears as a 
title attribute; this would be the default.  If you type text and 
select external link, the text is treated as a URL to the appropriate 
documentation and the appropriate links are rendered.

It makes more sense to me to use title attributes for the 
descriptions. These should theoretically even work for select boxes.

Because whups is so highly configurable, every site will have its own 
set of types, states, priorities, and attributes.  Because these 
values are site-specific, the help text should also be site-specific.

Example implementation: beside the display and input widget for 
built-in values (state, priority, summary, etc) show a help link if 
the administrator as assigned a URL to that variable.  Provide a 
general link to attribute definitions.

On input, this would look like:
* State [ 3 - Implement   \/] (?)

On display, this would look like:
   Type Enhancement (?)
   State 5 - Finished (?)

The ? are links to the administrator-defined help pages for each of 
those.  The administrator would be able to configure help links for 
each of the following built-in values:

Summary
Queue
Queue Version
Type
State
Priority
Owners
Requester
Created
Due
Updated
Assigned
Resolved
Attachments