Make small improvement to SDL_PropertiesID documentation

[TheMadman]

Description

Some places in the documentation (e.g. SDL_CreateProcessWithProperties) will show that the function takes an SDL_PropertiesID argument, then link to the SDL_PropertiesID type which helpfully informs you that it is an int. That's it.

I added links to places that can help people get started with understanding how to create property groups that can be passed to these functions, in case people end up at this page via some other API documentation that assumes they understand property groups already.

[TheMadman]

😵‍💫

[slouken]

This seems useful, but maybe there's a better way to link to the categories so it'll work from the headers. @icculus, thoughts?

[icculus]

So the category docs are linked from that page already, but it's probably too subtle to be useful.

We could add a wiki-only section (anything in a ## Whatever String We Want section will be preserved on the wiki and not migrate to the headers, as long as the title isn't a standard thing like "Remarks", etc).

[icculus]

Something like:

## Using Properties

While this just looks like an integer to the application, SDL properties are actually
key/value stores that can manage sets of information with multiple datatypes.

Please refer to [CategoryProperties](CategoryProperties) for all the details.

[icculus]

I went ahead and did this in libsdl-org/sdlwiki@67e2c99.