STYLE: Document SlicerWizard/TemplateManager.py

Add documentation for the public API of
SlicerWizard.ExtensionDescription. Also, make some minor code
improvements suggested in the course of writing the same:

• Allow clear() to take an optional name of a single attribute to remove
• Accept any object with a callable write() method as a stream in
  write()

Also, because we want to use it here, add a new reST role when
generating the documentation to create links to the Slicer wiki. (This
is useful because (1) it will allow us to later enhance the same to
select the appropriate version's documentation, and (2) the full links
are bloody long; way over our desired 80 character limit.)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22997 3bd1e089-480b-0410-8dfb-8563597acbee