Fix docs for TakeSourceScreenshot

[BarryCarlyon]

Revise the wording of TakeSourceScreenshot to note that if you specify a scene name, you get the scene.

Sorta #383

[dodgepong]

This is because scenes are sources. Perhaps the wording should say something like:

@param {String} `sourceName` Source name. Note that, since scenes are also sources, you can also provide a scene name.

Or perhaps more simply:

@param {String} `sourceName` Source (or scene) name.

[BarryCarlyon]

Yeah that wording is better. Will revise