[css3-lists] some comments

Simon Montagu and I went over http://dev.w3.org/csswg/css3-lists/ and
noticed a couple of fairly minor problems:

1. Example 13 ("visual rendering of the effect that ‘marker-side’ can have
on a list") is rendered differently in Chrome and Firefox. (In Chrome, the
bullets are adjacent to the left or or right margin; in Firefox, they are
adjacent to the left or right side of the item text.) Since the example is
meant to show the rendering required by the spec, we think it should render
as intended by the spec without relying on the capabilities of the
displaying browser.

2. Section 10 (Sample style sheet for HTML) currently says things like
padding-left and margin-right. These obviously only work for lists with
direction:ltr. These could be fixed by using :dir(ltr/rtl) pseudo-class
defined in Selectors level 4, except that Lists level 3 probably can't make
use of things in Selectors level 4. But at the very least a comment should
be added to the sample style sheet saying what is actually needed.