"why does my set-fontset-font call not work for symbols" is a common
question, and it's because 'use-default-font-for-symbols' is not
discoverable enough.
* doc/emacs/mule.texi (Modifying Fontsets): Mention it here.
* doc/lispref/display.texi (Fontsets): And here.
* src/fontset.c (Fset_fontset_font): And most importantly here.
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