Add doc comments for credentials (#355) #762

Job	Run time
lint	1m 13s
Build aarch64-apple-darwin target	2m 27s
Build x86_64-apple-darwin target	4m 29s
Build x86_64-unknown-linux-gnu target	3m 25s
msrv (ubuntu-latest, stable)	3m 10s
test (ubuntu-latest)	2m 59s
Build x86_64-pc-windows-msvc target	5m 8s
test (macos-latest)	2m 49s
Build x86_64-unknown-linux-musl target	5m 39s
test (windows-latest)	5m 20s
rust-test-vectors	4m 28s
kotlin-acceptance-testing (ubuntu-latest)	1m 12s
kotlin-acceptance-testing (macos-latest)	57s
kotlin-acceptance-testing (macos-12)	2m 35s
kotlin-acceptance-testing (windows-latest)	1m 47s
kotlin-deploy-snapshot	1m 40s
	49m 18s

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