x/tools/cmd/godoc: Possible improvements

[ianlancetaylor]

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.

[griesemer]

Comment 1:

Owner changed to @adg.

[mewmew]

Comment 2:

> * Provide some way to automatically know which types implement which interfaces,
ideally across packages.
The following package may serve as inspiration.
https://github.com/dominikh/implements

[rsc]

Comment 3:

Labels changed: added go1.3maybe.

[rsc]

Comment 4:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 5:

Labels changed: added repo-tools.

[bradfitz]

Item 2 seems to be happening in #18342

[mvdan]

Item 3 (a restricted version of it) is at #20131, accepted as a proposal.