[css-fonts-4] linkable definition of palette-mix() function (not <palette-mix()> type)

[foolip]

<palette-mix()> with angle brackets around it is used in the definition of font-palette's value, and I take the angle brackets and URL to mean it's a type, no the palette-mix() function itself.

I would like there to be a linkable definition of the function, similar to for color-mix():
https://drafts.csswg.org/css-fonts-4/#funcdef-palette-mix (does not exist)
https://drafts.csswg.org/css-color-5/#funcdef-color-mix (exists)

I suspect that it did work in @drott's #9282, but @cdoublev's #9381 removed it as duplicate. To avoid flipping back and forth, I'm filing an issue instead of sending a PR.

[cdoublev]

I think palette-mix() should be defined (wrapped in <dfn>) here.

This is the function definition pattern I think we should be aiming for:

The <dfn>foo()</dfn> function blah bla.

<pre class=prod>
  <<foo()>> = foo( ... )
</pre>

#2921 (comment)

[svgeesus]

I agree with @foolip that we should have a linkable definition. @cdoublev would you like to make a PR to define it?

[cdoublev]

Yes, sure. #9976