[css-values] Tweak wording to make it clearer that fullwidth glyphs are usually monospace. (Fix #357)

Author: fantasai

css-values/Overview.bs

@@ -980,8 +980,8 @@ Font-relative lengths: the ''em'', ''ex'', ''ch'', ''ic'', ''rem'' units</h4>
 			Equal to the used <a>advance measure</a> of the “水” (CJK water ideograph, U+6C34) glyph
 			found in the font used to render it.
 
-			This measurement is an approximation
-			(and in monospace fonts, an exact measure)
+			This measurement is a typically an exact measure
+			(in the few fonts with proportional fullwidth glyphs, an approximation)
 			of a single <a href="http://unicode.org/reports/tr11/#Definitions">fullwidth</a> glyph’s <a>advance measure</a>,
 			thus allowing measurements based on an expected glyph count.