This has been mentioned but I can't find a ticket, so I figured I'd give it is own thread, lest it be lost. Simply put, when the user hovers over a command, a short tool-tip/blurb will be displays along with the usual information, such as cost and requirements.
I figure there would be two ways to specify it in XML: either in the producing unit's command, or the produced unit itself.
1:
<command>
<type value="produce"/>
<name value="train_guard"/>
<image path="../guard/images/guard.bmp"/>
<tool-tip value="Heavy melee warrior. Slow speed, highly resistant, and good attack to ground units."/>
</command>
2:
<parameters>
...
<tool-tip value="Heavy melee warrior. Slow speed, highly resistant, and good attack to ground units."/>
</parameters>
The benefit for the first one (in the command) is that you can have different values for morph and produce, and it could also be made more generic (for other commands), e.g.:
<!-- produce command-->
<tool-tip value="Heavy melee warrior. Slow speed, highly resistant, and good attack to ground units."/>
<!-- morph command-->
<tool-tip value="With advanced training and heavy armor, a Swordman gains strength and resilience at the cost of speed."/>
<!-- attack command with a superfluous description-->
<tool-tip value="A heavy sword attack for massive damage to ground units."/>
...and if we want to get a little crazy...
<!-- effect description-->
<tool-tip value="The poison coursing through this unit's veins reduces speed and causes damage over time."/>
<!-- armor type-->
<tool-tip value="Teflon armor is highly resistant to sticky damage, but more susceptible to abrasion attacks."/>
... and the benefit of the second (specified in parameters) would be that you don't have to specify it in multiple places if your unit has more than one way of coming into existence.
Would it be better to read it from a text file somewhere? Would it be possible to make it multilingual?
My thoughts for that would be that you'd have multiple text files, and the engine would look for a language abbreviation in the file name. For example:
Heavy melee warrior. Slow speed, highly resistant, and good attack to ground units.
Combatiente pesado mano a mano. Baja velocidad pero alta resistencia y buen ataque contra unidades terrestres.
This might get rather cluttered, in which case a /units/guard/text folder would come in handy (but of course that's up to the modder).