internal/router: add some more comments
Merge pull request #43 from rogpeppe/016-bulk-include-comments

internal/router: add some more comments

As suggested by frankban in https://github.com/juju/charmstore/pull/39.

I started trying to add some sort of example to the FieldIncludeHandler
doc comment, but didn't do very well, so fell back to just pointing
to the best example we have.