Feedback on page SDL3/SDL_GetDisplays

[icculus]

• The note about freeing the results shouldn't be in the \returns section, but in general remarks. (We should clean this out across all the headers as we notice it...I should add it to documentation-rules, too).
• Does the SDL3 API promise this list is sorted with the displays from left-to-right positioning (except index 0, which is the primary display)? I see the code for SDL_GetDisplayBounds does, if the backend doesn't provide its own implementation, so I wasn't sure if these things were enforced across backends, too. If so, we should document this, as it's a useful property.
• The code example is useless, let's drop it.