Clarify platform-specific details for `Viewport` positioning #14426

Job	Run time
Check wasm32 + wasm-bindgen	2m 51s
Check wasm32+atomics	1m 56s
Format + check	2m 35s
cargo-deny aarch64-apple-darwin	40s
cargo-deny aarch64-linux-android	40s
ios	28s
android	24s
cargo-deny i686-pc-windows-gnu	48s
Run tests	2m 39s
cargo-deny i686-pc-windows-msvc	41s
cargo-deny i686-unknown-linux-gnu	43s
Check Windows	2m 4s
cargo-deny wasm32-unknown-unknown	41s
cargo-deny x86_64-apple-darwin	43s
cargo-deny x86_64-pc-windows-gnu	40s
cargo-deny x86_64-pc-windows-msvc	42s
cargo-deny x86_64-unknown-linux-gnu	40s
cargo-deny x86_64-unknown-linux-musl	43s
cargo-deny x86_64-unknown-redox	39s
	21m 17s

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