PEP 751: address some minor user feedback

[brettcannon]

Made it explicit that dependency-groups represents the groups users are expected to be able to set via a tool's UI. Also made clear what extras and dependency_groups should be set to by default.

• 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--4330.org.readthedocs.build/

[brettcannon]

I'm letting @pfmoore decide if this is worth merging.