[cssom-1] clarify resolved value of 'line-height: normal'.

Closes #3749.

Author: emilio

cssom-1/Overview.bs

@@ -2947,11 +2947,13 @@ as follows:
  <dt>'box-shadow'
  <dt>'caret-color'
  <dt>'color'
- <dt>'line-height'
  <dt>'outline-color'
  <dt>A <dfn export>resolved value special case property like 'color'</dfn> defined in another specification
  <dd>The <a>resolved value</a> is the <a>used value</a>.
 
+ <dt>'line-height'
+ <dd>The <a>resolved value</a> is 'normal' if the <a>computed value</a> is 'normal', or the <a>used value</a> otherwise.
+
  <dt>'block-size'
  <dt>'height'
  <dt>'inline-size'