/fyi @mdo There might be prettier/better ways of explaining these,
        but for the sake of having accurate docs for tomorrow,
        this should do in a pinch.

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Refs #18110

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Refs #18459, #17178
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https://bugs.webkit.org/show_bug.cgi?id=151933

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* `.font-normal` is too generic. Rename it to `.font-weight-normal` for clarity.
* Rename `.font-bold` to `.font-weight-bold` so as to parallel `.font-weight-normal`.
* In docs, gloss "weight" term in relation to fonts for the benefit of non-typographiles.

Refs #18433

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https://bugzilla.mozilla.org/show_bug.cgi?id=1139853
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https://bugzilla.mozilla.org/show_bug.cgi?id=1230801

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