We prohibit std::atomic in favor of AtomicWord, but it's not clear from the error message in https://github.com/mongodb/mongo/blob/master/src/mongo/tools/mongo_tidy_checks/MongoStdAtomicCheck.cpp#L57-L59 why we do so. Furthermore, we should document how to use AtomicWord safely.

AC:

• The error message points to documentation clearly explaining why std::atomic is prohibited.
• There is a "new engineer"-style doc on Confluence clearly explaining how to use AtomicWord safely.