This change brings the multisite specific `promote` user action up-to-speed with the single-site one, by adding:
* capability checks where appropriate
* a `none` check on `$role` to set it to an empty string
It also updates the inline documentation of the single-site `promote` user action in `users.php`, to match the suggested additions to the multisite file.
Props ignatiusjeroe, jeremyfelt, johnjamesjacoby, pratiklondhe, shanemuir, sudipatel007, techpartho.
Fixes#61100.
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