Spec change: clarify namedtuple member rules

[yangdanny97]

Hi, I'd like to request a review for python/typing#1979

Discussion: https://discuss.python.org/t/adding-namedtuple-member-naming-rules-to-the-typing-spec/88882

Nobody said anything, so I hope this isn't a controversial change.

Signoff:

• Carl
• Eric
• Jelle
• Jukka
• Rebecca