Avoid problem of old sphinx reporting an error on lines starting from :

See http://sourceforge.net/tracker/?func=detail&aid=3484857&group_id=38414&atid=422030
Hack to stop parsing of problematic CvStatModel::train definition
Fix warnings about missing parent for class members
Fix several inconsistent declarations identified by check_doc2 script
Add support for enums into rst ocv domain
Whitelist functions/structs in check_docs2 that are impossible to fix in 2.4
Merge pull request #503 from taka-no-me:fix_docs