io/fs: clarify additional File interface docs

io/fs: clarify additional File interface docs

Emphasize ReadDirFile. It isn't really optional,
and all filesystems have at least one directory (".").

The remaining two additional interfaces are optimizations.
Call them that.

Fully qualify package package io identifiers.

Change-Id: Ibc425a5dfd27e08c2c10c353f780e4a6304cfd87
Reviewed-on: https://go-review.googlesource.com/c/go/+/296390
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>