[mediaqueries] Add cross-reference to Units section?

For the benefit of folks who get linked directly to
https://drafts.csswg.org/mediaqueries/#width , and since 'width' &
'height' are the only media features that involve lengths, consider
adding a cross-reference to https://drafts.csswg.org/mediaqueries/#units
into §4, §4.1, or Example 26, to point out the special handling of
relative length units (em, rem, etc.).

Cheers,
Chris
--
http://chrisrebert.com
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