Improved documentation for builtin symbols

Now that we have more visible documentation in the editor, it's conspicuous how much the documentation is missing for a lot of builtin symbols. It would be good to add documentation (which just means haddock in the codeworld-base module) for all these definitions.

At the same time, I should rewrite the existing documentation to be clearer about function arguments. We should adopt this style for the first sentence of every function:

@thickArc(angle1, angle2, radius, thickness)@ is a @Picture@ of an arc
between the given angles, with the given radius and line thickness.

The documentation can go on to give examples and such, but that first sentence clarifies what each argument means in a precise way.