Add demangle to glossary

Add a discussion of mangled symbols to the glossary and a mention of how to use `c++filt` to demangle them. Make this relevant by noting that even the latest and greatest C++ libraries have to deal with this like https://github.com/abseil/abseil-cpp/commit/dca2eb50f9f5ee378395ee020953dc20a2a84c97