Add note about stdout exporter (#1491) #4024

Job	Run time
coverage	7m 58s
docs	5m 40s
cargo-deny	33s
external-types (opentelemetry)	1m 15s
external-types (opentelemetry-sdk)	1m 23s
lint	20m 13s
msrv	3m 36s
non-default-examples (opentelemetry-otlp/examples/external-otlp-grpcio-async-std)	5m 15s
test (stable)	16m 16s
non-default-examples (opentelemetry-otlp/examples/basic-otlp)	46s
test (beta)	17m 11s
	1h 20m 6s

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