update msLayerObj.generateSLD() docu

[ddegasperi]

In the documentation it's not clear that the layer has to have status MS_ON to successfully generate the SLD with the function "generateSLD" of the msLayerObj (the same applies to msMapObj).

The example "PHP/MapScript Example that Generates an SLD from a Mapfile" on following URL: http://mapserver.org/ogc/sld.html#php-mapscript-example-that-generates-an-sld-from-a-mapfile should also contain a hint about it (before calling generateSLD):

// force visibilty of the layer
$oLayer->set('status', MS_ON);

[havatv]

Fixed for 6.0 and trunk documentation in r12050. Documents updated: mapscript/php/index, mapscript/mapscript, ogc/sld.