Add docs to custom vertex attribute example #56198

Job	Run time
build (windows-latest)	12m 29s
check-bevy-internal-imports	4s
build (ubuntu-latest)	7m 56s
check-doc	12m 47s
build (macos-latest)	5m 31s
check-missing-examples-in-docs	28s
ci	4m 39s
miri	11m 21s
run-examples-macos-metal	6m 21s
toml	2m 5s
typos	8s
check-missing-features-in-docs	27s
markdownlint	1m 2s
check-compiles-no-std	2m 6s
check-compiles	7m 31s
msrv	1m 45s
build-wasm	1m 36s
build-wasm-atomics	1m 56s
	1h 20m 12s

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