PEP 747: Explicitly define "valid type expression".

[rchen152]

When I was reading the PEP, it was unclear to me whether the rule on what expressions can implicitly evaluate to TypeForm applied to the rest of the PEP. This PR suggests pulling that rule out into its own section and linking to it the first time "valid type expression" is used, to make it clear that this definition applies throughout.

• Change is either:
  • To a Draft PEP
  • To an Accepted or Final PEP, with Steering Council approval
  • To fix an editorial issue (markup, typo, link, header, etc)
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

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📚 Documentation preview 📚: https://pep-previews--4484.org.readthedocs.build/

[hugovk]

Ready to merge?