Bug#864033: stretch: Document that "Unescaped left brace in regex is deprecated" is harmless

[Adrian Bunk <bunk@debian.org>]

Package: release-notes
Severity: normal

Since #863479 won't be fixed in stretch, it should be documented in
4.5. Possible issues during upgrade that the following warnings
during an upgrade are harmless if they happen due to the order of
packages upgraded:

Unescaped left brace in regex is deprecated, passed through in regex; marked by <-- HERE in m/^(.*?)(\\)?\${ <-- HERE ([^{}]+)}(.*)$/ at /usr/share/perl5/Debconf/Question.pm line 72.
Unescaped left brace in regex is deprecated, passed through in regex; marked by <-- HERE in m/\${ <-- HERE ([^}]+)}/ at /usr/share/perl5/Debconf/Config.pm line 30.