cmd/doc: don't print a declaration twice

cmd/doc: don't print a declaration twice

That can occur if we have -u set and there is an upper- and lower-case
name of the same spelling in a single declaration.

A rare corner case but easy to fix.

Fix by remembering what we've printed.

Fixes #21797.

Change-Id: Ie0b681ae8c277fa16e9635ba594c1dff272b8aeb
Reviewed-on: https://go-review.googlesource.com/78715
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>