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
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documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc