Add doc comments for credentials #756

Job	Run time
lint	1m 15s
msrv (ubuntu-latest, stable)	3m 11s
Build aarch64-apple-darwin target	2m 10s
test (ubuntu-latest)	3m 7s
Build x86_64-apple-darwin target	6m 28s
test (macos-latest)	3m 14s
Build x86_64-pc-windows-msvc target	5m 43s
test (windows-latest)	3m 8s
Build x86_64-unknown-linux-gnu target	3m 29s
Build x86_64-unknown-linux-musl target	5m 50s
rust-test-vectors	2m 42s
kotlin-acceptance-testing (ubuntu-latest)	57s
kotlin-acceptance-testing (macos-latest)	1m 11s
kotlin-acceptance-testing (macos-12)	2m 37s
kotlin-acceptance-testing (windows-latest)	1m 52s
kotlin-deploy-snapshot	1m 50s
	48m 44s

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