Improve "Custom Typeface" "import url" instructions

The existing instructions under "import url" for "Custom Typefaces" works well enough if user is importing Google fonts. However, it is too ambiguous if user is uploading their own font file into the project.
The instructions should specify that the "import url" should be a .css file, which includes @font-face import url instructions. If user directly serves up a font file instead, the font will not be loaded correctly into the stylesheet.

Thanks for the feedback, @Christopher_Hugentobler. I've logged it as a feature request. :+1: