Better scaladoc for extension methods for Column and Query.

- Link to the extension method classes in the scaladocs for `Column` and
  `Query`.

- Mark implementation details of the extension method traits as
  `protected[this]` where applicable to remove them from the generated
  documentation. Unfortunately, this is not possible for the constructor
  parameters of value classes.

Fixes issue #846.
Merge pull request #863 from slick/tmp/document-implicits

Better scaladoc for extension methods for Column and Query.