Further overhaul of the additional documentation section

Further overhaul of the additional documentation section

Building on top of Ben's patch, this includes the following changes:

* Shared library documentation is called out as a special case of the
  general rule that documentation should be packaged separately if it's
  large, recommending installing documentation of any appreciable size in
  the development package or a separate documentation package rather than
  the shared library package.

* The naming convention for documentation packages (package-doc or
  package-doc-language) is explicitly stated (but is not a requirement).

* package-doc is allowed to put documentation in either
  /usr/share/doc/package or /usr/share/doc/package-doc, but the former is
  preferred.  package-doc still has to install its own mandatory
  documentation files (changelog, copyright) in
  /usr/share/doc/package-doc.

* The paragraph about /usr/doc is removed.

* Some reorganization and wording tweaks to hopefully make things clearer.