What can the minidisplay port on the MacBook Air do?
I am not aware of any manual that contains this information but here is a breakdown, since this is one of the big reasons I got a MacBook originally.

The mini display port allows you to add an additional monitor to your MacBook. Support for this is built into the OS directly so you simply connect your external monitor using the correct MD/VGA or MD/HDMI or MD/DVI connector if the monitor does not have a display port and then you can configure how the displays work. This is done under the Displays option in System Preferences.

From the screen shot below you can see I am running a second monitor on my MacBook Pro at the moment and I have set the second monitor to be to the left of the main one by simply dragging it's rectangle to where I want it.

You can also move the menu to a different screen by grabbing the white strip on the primary screen and dragging it to another screen you want it on.

Each display can have it's own separate resolution and even separate backgrounds. The relevant preference panes will automatically show a dialogue on each screen if it supports multiple displays, for example the Desktop &amp; Screensaver will show on both screens.

You can also choose whether you want to mirror the display or use it as a second screen. In mirror mode it simply mirrors everything on your main display.