Clarify platform-specific details for Viewport positioning

[aspiringLich]

On Wayland (and android apparently) it is not possible for applications to position their windows or request the window position (for some reason). This is not specified in the documentation.

proof: winit::set_outer_position, winit::outer_position, winit::inner_position all specify that Android / Wayland are not supported.

• Maybe closes with_position method of ViewportBuilder does not seem to work on wayland #4469
• the lack of support makes me sad. I believe this makes some items in Tracking issue: Misc improvements to multi-viewports #3556 impossible on Wayland. Oh well!

[lucasmerlin]

Nice, thank you!

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5715-viewport-position-docs
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[lucasmerlin]

Ah, should've realized in the first review, we should mention that this is eframe specific, since it might be different for someone e.g. using bevy_egui. I'll apply the suggestions and merge this now, thanks!