Rhino 6 documentation bugs

[Andrew_Nowicki]

@KelvinC

Instead of adding a wish for undo, I added a wish for combining AddGuides and RemoveGuides into a single command. I think that is more useful than supporting undo.

I agree. Rhino already has too many similar commands: Tweedledum and Tweedledee commands

In my opinion, it does not matter whether Undo and Redo commands ignore guides, but Purge command should be able to purge everything, including guides. At present, the Purge command ignores guides. If Undo, Redo, and Purge commands ignore guides, this fact should be mentioned in documentation.