Clarify that findfont & _find_fonts_by_props return paths.

[anntzer]

Especially the previous wording for _find_fonts_by_props ("find font families") was somewhat confusing, as "font families" are strings.

PR summary

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[oscargus]

Not sure how it should be milestoned in these release times, but apart from that looks good! (If you know, @anntzer, milestone and self-merge.)