Pull up common doc_* handlers into the base handler class

Avoids duplication if plugins want to have their arguments
documented the same as the builtin commands.

The slight downside is that I had to add empty methods to the
operation doc handler because they don't have synopsis/options
sections.  The plus side is that plugins don't have to write
any custom doc handlers to get automatic argument documentation.
Merge branch 'extract-base-doc-handler' into develop

* extract-base-doc-handler:
  Pull up common doc_* handlers into the base handler class