regexp/syntax: add note about Unicode character classes

As proposed on golang-nuts:
https://groups.google.com/g/golang-nuts/c/M3lmSUptExQ/m/hRySV9GsCAAJ

Includes the latest updates from re2's mksyntaxgo:
https://code.googlesource.com/re2/+/refs/heads/master/doc/mksyntaxgo

Change-Id: Ib7b79aa6531f473feabd0a7f1d263cd65c4388e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/264678


Reviewed-by: Russ Cox <rsc@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>