Clarify meaning of "best" quality (bitrate vs. framerate, etc.) in readme

[ToaKraka]

Fixes #28307

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Description of your pull request and other information

Clarifies (in the readme, not in any actual code) the method by which "best" or "worst" quality is determined, for users who are confused when the highest-framerate video is not downloaded by default

[dstftw]

How youtube-dl implements this logic is an implementation detail and may change any time. Moreover this logic may vary from extractor to extractor.