Update selected RFC 2822 references to RFC 5322
RFC 2822 was obsoleted by RFC 5322 in 2008. This updates references
to use the current standard in documentation, docstrings, and comments.
It preserves RFC 2822 references in legacy API components to maintain their
historical context.
RFC 822 → RFC 2822 → RFC 5322 progression is explained where relevant.
In some places specific sections of RFC are referenced where it seems helpful.
Scout rule was applied in some places and RFC mentions format was
normalized in doc strings and comments.
