ISSUE-136: add note upon first use of 'c' length metric [DFXP 1.0 (Editorial)]

ISSUE-136: add note upon first use of 'c' length metric [DFXP 1.0 (Editorial)]

http://www.w3.org/AudioVideo/TT/tracker/issues/136

Raised by: Glenn Adams
On product: DFXP 1.0 (Editorial)

because DFXP introduces the 'c' length metric, and many readers (that are familiar with CSS) may be puzzled by it, ensure that a note or forward reference appears at its first usage in spec, which appears to be 8.2.10;