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.

This should not change the output of any of the existing reference docs.