docs: improved documentation of the extra/ extensions

Simplified, unified style, added some undocumented options.

NOTE: Footnotes/UNIQUE_IDS stays mostly undocumented
docs: update codehilite documentation
docs: update header_id documentation
docs: update meta_data documentation
docs: update nl2br documentation
docs: update sane_lists documentation
docs: update smarty documentation
docs: update toc documentation
docs: update wikilinks documentation
docs: capitalization fixes, plural fixes, small wording improvements
Merge pull request #288 from lahwaacz/master

docs: improved documentation
Cleaned up some minor issues with recent changes to docs.
Upped version to 2.4 final.