Currently there are "On AC" and "On Battery" option categories. It would terrific if an "On Dock" category could be added so that power settings could be applied differently to just being "On AC".
I guess we could check for the is-docked variable in UPower and if that's the case show an additional tab with specific settings. Extending the existing table with yet another column might be a bit too much (visually). Either way, this won't be worked on very soon for sure. Feel free to submit a patch though or motivate others by setting a bounty on the bug.
Guess this is nothing we'll do very soon so for now you have to live with the distinction of "On Battery" and "On AC" (the latter also includes the Dock).