Some improvement we could make to godoc.  Split these up into separate issues if it's
easier.

* Add the ability for godoc-specific comments.  This would not be markup as such, but
would be ways to organize the presented docs.  For example, a way to indicate which
names in a package are important and which are not.  The important names would appear
first.  The unimportant ones would be hidden by default.

* When the text has a capitalized word that is the same as an exported name in the
package, link the text to the docs for that name.

* Provide some way to automatically know which types implement which interfaces, ideally
across packages.