Define read-only and write-only properties in the documentation

[falkoschindler]

Currently the documentation doesn't tell if a property is meant to be read or written to. This can be confusing if you, e.g., want to set

output.active = true

and it doesn't do anything because active is read-only and you should use activate() instead.

An extra "access" column for property tables with "R", "W" or "RW" should do.

Update: I noticed my example is invalid because active can is read+write. But my documentation request still holds.