Just some minor typo and formatting fixes to the geshi-doc file.
Typo and spelling fixes
For places where apostrophes should be literal apostrophes because they're referring to that exact character in a programming language, undo their styling as curly quotes
Add periods to end of sentences which are closed only with smileys
Fix a broken footnote link for MediaWiki
Update the download link to where it's currently redirecting to
Just some minor typo and formatting fixes to the geshi-doc file.