The existing PR can be re-targeted to merge to a maintenance branch (I'd be inclined to merge manually, myself, but will have to check the current devguide to make sure that's still allowed).

A new PR can be made for the non-documentation fix for master.

My thought is that a PR is more about the patch than about the workflow; a different patch can be handled in a separate PR, and review & discussion are used to determine which PR should be applied where.