all: add a few more godoc links

Over the past few months as I read the standard library's documentation
I kept finding spots where godoc links would have helped me.
I kept adding to a stash of changes to fix them up bit by bit.

The stash has grown big enough by now, and we're nearing a release,
so I think it's time to merge to avoid git conflicts or bit rot.

Note that a few sentences are slightly reworded,
since "implements the Fooer interface" can just be "implements [Fooer]"
now that the link provides all the context needed to the user.

Change-Id: I01c31d3d3ff066d06aeb44f545f8dd0fb9a8d998
Reviewed-on: https://go-review.googlesource.com/c/go/+/508395
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

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