go/doc: skip escaping comments in pre-formatted blocks
CL 150377 made the change of converting smart quotes to their html escaped entities
for ToHTML, and to unicode quotes for ToText. But for ToText, the change
converted the quotes in pre-formatted text too.
This fixes that behavior to not touch any text in pre-formatted blocks, which also
makes the behavior consistent with ToHTML.
Fixes #29730
Change-Id: I58e0216cbdbe189d06d82147e5a02b620af14734
Reviewed-on: https://go-review.googlesource.com/c/162922
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>