Since I think I know how to do it, easily, I will try to derive the 2.7 patch.
In Matching Characters, I think
"The following predefined special sequences are available:"
should be expanded to
"The following predefined special sequences are a subset of those available. The equivalent classes are for bytes patterns. For a complete list of sequences and expanded class definitions for Unicode string patterns, see the end of Regular Expression Syntax."
(with section reference markup).
Note to myself. /bytes/byte string/ for 2.7.
While the changes all look innocuous to me with respect to building the docs, I am curious if you have tried to rebuild the HOWTO (if you have the tool chain, which I do not).