half-leading trait

The specification text in §6.6.7 fo:inline (and probably also
fo:inline-character
and fo:character) should have language analogous to that found in §6.5.2
since line-height applies to fo:inline (and other FOs):

*Trait Derivation:*

The .minimum, .optimum, and .maximum components of the *half-leading* trait
are set to 1/2 the difference of the computed value of the
*line-height* property
and the computed value of the sum of the *text-altitude* and
*text-depth* properties.
The .precedence and .conditionality components are copied from the
*line-height* property.

*Note:*

The usage of the half-leading is described in *4.5 Line-areas*
<https://www.w3.org/TR/xsl11/#area-line>.
In addition, the language in §4.5 should be clarified with respect to use of
 *half-leading*. In particular, the paragraph starting with "The
*expanded-rectangle* ..." uses *half-leading* twice, but in the first case,
it is referring to the half-leading of an inline-area, and in the second
case it is referring to the half-leading of a line-area.

I would also suggest the following regarding this paragraph:

   - split into two paragraphs, starting a new paragraph at "The
   *expanded-nominal-requested-line-rectangle* ...";
   - change "The *expanded-nominal-requested-line-rectangle* ..." to "The
   *expanded-nominal-requested-line-rectangle* for a line-area ...";
   - clarify that the term *expanded-rectangle* and the word *outside*, as
   used in the existing paragraph, can actually mean *contracted rectangle*
    and *inside* in the case that half-leading is negative;

I'm sending this for archival purposes, on the off chance that someone
updates XSL-FO in the future, or perhaps just to start a conversation on
ambiguities around the concepts of line height and line stacking.