Document addons:available suffix

[hvacengi]

Apparently when I revised the addon system I never documented that addons:available("name") was a new suffix to check availability of both the addon itself and the associated mod/feature. Because the addon labels are added as suffixes to the addons list based on an asembly walk, calling addons:camera:available without my camera mod installed will throw an exception, while addons:available("camera") will still return false.

Not extremely pressing, but it is a small documentation change that we should make users aware of.