x/tools/godoc: index headers don't shorten verbose inline type definitions

[dsnet]

(moved from google/go-cmp#135)

When a declaration includes anonymous types, the index can format the declaration as something completely unreadable. For example, see the Reporter type in the index list of cmp.

Note that affects gddo and x/tools/godoc, but not cmd/go/doc since I fixed this in 2016. The fundamental problem is that all the godoc-like tools do not share much logic in terms of how it renders the front-end, so improvements to one are not benefited by all.

[dmitshur]

This is already fixed in the documentation rendering done by discovery site (tracked in issue #33654):

What remains here is to backport the fix to x/tools/cmd/godoc.