Make docs fallback font 'serif', rather than 'sans-serif'

[cllns]

The fallback font for a serif font should also be serif, not sans serif.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) soon.

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[nagisa]

Why remove Helvetica, Arial and friends?

[cllns]

@nagisa because they're sans-serif fonts. If "Source Serif Pro" cannot load, we should load another serif font, not a sans-serif one.

[steveklabnik]

I agree with this conceptually, but the gap between "Source Serif Pro" and serif is pretty huge. Maybe put another one or two in there too? http://www.smashingmagazine.com/2009/09/22/complete-guide-to-css-font-stacks/

[iKevinY]

If we were to remove the sans-serif fallbacks in favour of serif fonts, I think a Georgia-based stack would work pretty well. It supposedly has good compatibility and looks somewhat similar to Source Serif Pro.

font-family:"Source Serif Pro", Georgia, Times, Times New Roman, serif;

[cllns]

Good call! Just updated it to the Georgia stack as the fallback.

[alexcrichton]

r? @steveklabnik

[steveklabnik]

@bors: r+ 45f6672 rollup