[CSS21] list marker positions: ltr vs rtl

Eira has been working on bidi tests for CSS2.1 and ran into a spec
problem when working on a test for lists.

http://www.w3.org/TR/CSS21/generate.html#propdef-list-style-position

We have a normative statement here that says

   # In right-to-left text, the markers would have been on the right
   # side of the box.

The spec clearly /implies/ that markers in ltr elements should be
on the left and markers in rtl elements should be on the right,
but it doesn't anywhere say so explicitly. So I propose we clarify
the spec as follows:

Append to

   # outside
   #  The marker box is outside the principal block box. CSS 2.1
   # does not specify the precise location of the marker box.

the following

   | but does require that for elements whose 'direction' property
   | is 'ltr' the marker box be on the left side of the content and
   | for elements whose 'direction' property is 'rtl' the marker
   | box be on the right side of the content.

And absorb that previously-quoted statement into the example it's
referring to.

~fantasai