Add docs for `#[rpc]` #2968

Job	Run time
doc-lints	41s
clippy	1m 58s
godot-itest (macos)	1m 4s
rustfmt	10s
license-guard	27s
godot-itest (windows)	2m 8s
godot-itest (linux-full)	3m 6s
run-examples	1m 7s
unit-test	2m 12s
godot-itest (linux-features-experimental)	1m 56s
godot-itest (linux-4.1)	58s
godot-itest (linux-4.2)	1m 3s
godot-itest (linux-memcheck)	2m 25s
ci-status	3s
	19m 18s

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