Clarify "enable" for modus-themes-mixed-fonts

Stumbling upon the deprecation warning of modus-themes-no-mixed-fonts that was set to nil in my case, I was briefly confused by the meaning of modus-themes-mixed-fonts and its default value of nil. I personally like when docstrings say which value does what, e.g. in the "Non-nil to ..." format.

Just a minor wording suggestion. Not sure if it fits your style.