[3.13] gh-67022: Document bytes/str inconsistency in email.header.decode_header() and suggest email.headerregistry.HeaderRegistry as a sane alternative (GH-92900)

[3.13] gh-67022: Document bytes/str inconsistency in email.header.decode_header() and suggest email.headerregistry.HeaderRegistry as a sane alternative (GH-92900) #183083