Add documentation to ==>>

I had never used `==>>`, so I thought it would be nice to start looking
through its source, adding ScalaDoc comments as I went along.

This is  how far I got before I thought to check how many lines
were in the file. Adding documentation for the rest is a bit of a
daunting task, but I figured I might as well contribute what I had
already done.

:-\
Merge pull request #711 from ceedubs/docs-map

Add documentation to ==>>