FR: Improve documentation of createPluginFilter()

Just ran into the problem of having an undefined symbol: createPluginFilter() error because I’d mistakenly placed the definition of the function in a namespace.

The documentation of createPluginFilter states:

/** Somewhere in the codebase of your plugin, you need to implement this function
and make it return a new instance of the filter subclass that you’re building.
*/

This is a little misleading because the function must be defined in the global namespace, so not just “somewhere in the codebase”.

Perhaps the documentation could be updated to something like:

Somewhere in the global namespace of your plugin’s codebase, you need to implement this function and make it return a new instance of the filter subclass that you’re building.