Clarify public and private wiki permission levels

[HonkingGoose]

Why:

Closes #10075

What's being changed:

• Change the current line to "Edit wikis in public repos"
• Add a second entry for "Edit wikis in private repos" that only has Xs in write, maintain and admin.

Check off the following:

• I have reviewed my changes in staging (look for the latest deployment event in your pull request's timeline, then click View deployment).
• For content changes, I have completed the self-review checklist.

Writer impact (This section is for GitHub staff members only):

• This pull request impacts the contribution experience
  • I have added the 'writer impact' label
  • I have added a description and/or a video demo of the changes below (e.g. a "before and after video")

Question: Do we need another {% ifversion fpt %} on the private wiki line???

[ramyaparimi]

@HonkingGoose
Thanks so much for opening a PR! I'll get this triaged for review ⚡

[lecoursen]

Thanks @HonkingGoose! I'm going to apply one small suggestion and then merge. ⚡

[github-actions]

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