note PresentMode::Mailbox support for Wayland

[i509VCB]

Wayland is inherently a mailbox system and hence should be noted as always supporting Mailbox (at least on Vulkan where it's explicitly known, the Wayland EGL platform isn't too clear about the present mode).

Checklist

• [X] Run cargo clippy.
• [X] Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• [ ] Add change to CHANGELOG.md. See simple instructions inside file.

Description Note support for the Mailbox present mode on Wayland on Vulkan. Wayland is inherently a mailbox system and therefore all Vulkan implementations must report the mailbox present mode.