[css2] Review and moderate rewrite of this chapter.

Mostly organizational with a few changes:

- Added gentler example to explain :before/:after,
  rather than quotation marks right off the bat.
- Added counters() to 'content' property values.
- Added an aural example with url() to 'content' section
- Divided quotation marks section in two.
- Replaced an example with an informative note about
  high-quality typography quotes and table of ISO10646
  characters.
- Changed "number" to "integer" in automatic counters description.
- Added example of counters() (I used the existing text but
  turned it into an example).
- Cleared up marker-offset to refer to distance between box
  *borders* (after discussion with Hakon).
- Fixed smiley example (alignment of ascii art :after was wrong).

--HG--
extra : convert_revision : svn%3A73dc7c4b-06e6-40f3-b4f7-9ed1dbc14bfc/trunk%401339

Author: ianbjacobs