RenderDevice::poll return type changed from v0.12.1 to v0.13

In v0.12.1 `RenderDevice::poll` returned a `bool` where true indicated the queue was empty:
https://github.com/bevyengine/bevy/blob/22e39c4abf6e2fdf99ba0820b3c35db73be71347/crates/bevy_render/src/renderer/render_device.rs#L63

This changed with PR #11280 and `RenderDevice::poll` now returns `wgpu::MaintainResult::SubmissionQueueEmpty` instead when the queue is empty:
https://github.com/bevyengine/bevy/blob/abb8c353f49a6fe9e039e82adbe1040488ad910a/crates/bevy_render/src/renderer/render_device.rs#L64

`RenderDevice::poll` still indicates the old return type in its associated doc comment and no migration for the change was included in the PR or the recent guide for updating to v0.13.

A separate issue #11079 exists that may help with the new return type.

Start in crates/bevy_render/src/renderer/render_device.rs and compare the associated documentation with the v0.13 return value described in the issue. Then locate the v0.13 migration guide and add the missing guidance for callers; done means the API comment and migration documentation no longer describe the old boolean result.

Written by the indexing model from the issue text.