all: replace `` and '' with “ (U+201C) and ” (U+201D) in doc comments

all: replace `` and '' with “ (U+201C) and ” (U+201D) in doc comments

go/doc in all its forms applies this replacement when rendering
the comments. We are considering formatting doc comments,
including doing this replacement as part of the formatting.
Apply it to our source files ahead of time.

For #51082.

Change-Id: Ifcc1f5861abb57c5d14e7d8c2102dfb31b7a3a19
Reviewed-on: https://go-review.googlesource.com/c/go/+/384262
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>