doc: add C++ style comments to the style guide

Adds instructions on how to format C++ comments to the C++ style guide, as cpplint.py doesn't complain about C-style comments on the code and C++ style comments are the encouraged format.

There's a lot of C-style comments on the current code, and because of that new contributors may think C-style comments are the encouraged format for multi-line comments.

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc