Documentation and helper functions for methods in the parser

[dvd101x]

Hi, this is kind of embarrassing for me.

Recently I was reminded at #3441 that it's possible to use the methods of units in the parser.

Even though I've made objects that includes functions in the parser just to be called later, it just clicked for me that it's possible to run in the parser.

a = 1 m.           # 1 m
a.toNumber('mm').  # 1000

Maybe it's only a blind spot of mine, but I think a section in syntax stating that it's possible to use most methods in the parser or showing their use in their respective datatypes section in the parser would be a nice addition.

[josdejong]

Good point David. Addressed via 9b8e324.

Enjoy your newly discovered options :)