Here are my suggested help strings:
$string['firstpagetitle_help'] = 'The first page name is displayed in the navigation bar and at the top of the first page.';
$string['wikimode_help'] = 'The wiki mode determines whether everyone can edit the wiki - a collaborative wiki - or whether everyone has their own wiki which only they can edit - an individual wiki.'
$string['defaultformat_help'] = 'Currently only HTML format can be used for editing wiki pages. Additional formats will be available in future.';
$string['forceformat_help'] = 'If the format is forced (checkbox ticked), there is no option to choose a format when editing a wiki page.';
$string['diff_help'] = 'Selected versions of the page may be compared in order to find the differences.';