Add doc comments for credentials #760

Job	Run time
msrv (ubuntu-latest, stable)	3m 10s
test (ubuntu-latest)	2m 58s
Build aarch64-apple-darwin target	2m 51s
test (macos-latest)	2m 10s
test (windows-latest)	2m 58s
Build x86_64-apple-darwin target	5m 55s
Build x86_64-pc-windows-msvc target	4m 55s
Build x86_64-unknown-linux-gnu target	3m 32s
Build x86_64-unknown-linux-musl target	5m 43s
rust-test-vectors	2m 57s
lint	1m 12s
kotlin-acceptance-testing (ubuntu-latest)	1m 15s
kotlin-acceptance-testing (macos-latest)	54s
kotlin-acceptance-testing (macos-12)	2m 45s
kotlin-acceptance-testing (windows-latest)	1m 38s
kotlin-deploy-snapshot	1m 45s
	46m 38s

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