Update PHP docs to be consistent about rendered filename

[jonyo]

The "real" spec for a render, says it should not change the current fileType to match the rendered image file type, it should just return the filename when the render() method is called. If, as part of the render process, it uses a conversion (like OpenSCAD to STL), that is OK to change the file type. But the file type should always be for the current non-rendered 3d file.

This spec was developed over time, and some of the PHP docs for some methods state that the fileType could match the "converted" or the "rendered" file type, update all places in the code to reflect the latest spec (fileType only changes for converters).

[jonyo]

Note: this repo hasn't had much change but we are still around! We just haven't had much need for change.

If anyone wants to help contribute this is a good issue to start with. 😃