Specify used regex syntax in clang-format documentation

I was writing a configuration file for clang-format, especially regexes for the IncludeCategories option and I was trying to use character classes like \w which failed.

In the documentation there is no mention of the regex syntax used and I had to look into the source code for llvm::Regex to find out it is using Posix ERE.
It would be nice to have a note in the documentation which mentions that Posix ERE is used.

The bugtracker is closed for new users so I hope someone from this list could file a bug against the documentation.