Doc comment updates for CoreTagHooks callback functions + Parser::setHook() & Parser...

Doc comment updates for CoreTagHooks callback functions + Parser::setHook() & Parser::setTransparentTagHook().

Added notes about the 'extended' return values from some of the hooks, which seems to be undocumented but is vaguely similar to what is documented for parser function callbacks. It doesn't look like a very safe interface, and could stomp on internal variables if one isn't careful. Needs to be better defined and doc'd.

setTransparentTagHook() seems to have been largely undocumented and is only used by the 'geoserver' extension currently. Is this considered obsolete? Should it be simply replaced by use of the frame & recursiveTagParse goodies, or is it still needed for something?