Add doc comments for credentials #751

Job	Run time
Build aarch64-apple-darwin target	2m 52s
Build x86_64-apple-darwin target	6m 34s
Build x86_64-pc-windows-msvc target	5m 17s
Build x86_64-unknown-linux-gnu target	3m 35s
Build x86_64-unknown-linux-musl target	5m 44s
lint	1m 13s
msrv (ubuntu-latest, stable)	3m 9s
rust-test-vectors	2m 46s
test (ubuntu-latest)	3m 17s
test (macos-latest)	3m 17s
test (windows-latest)	6m 10s
kotlin-acceptance-testing (ubuntu-latest)	57s
kotlin-acceptance-testing (macos-latest)	1m 4s
kotlin-acceptance-testing (macos-12)	1m 46s
kotlin-acceptance-testing (windows-latest)	1m 49s
kotlin-deploy-snapshot	1m 41s
	51m 11s

Provide feedback