Add doc comments for credentials #757

Job	Run time
lint	1m 18s
msrv (ubuntu-latest, stable)	3m 16s
Build aarch64-apple-darwin target	2m 53s
Build x86_64-apple-darwin target	6m 58s
Build x86_64-pc-windows-msvc target	5m 18s
Build x86_64-unknown-linux-gnu target	3m 31s
Build x86_64-unknown-linux-musl target	5m 47s
test (ubuntu-latest)	3m 4s
test (macos-latest)	2m 48s
test (windows-latest)	2m 48s
rust-test-vectors	2m 58s
kotlin-acceptance-testing (ubuntu-latest)	55s
kotlin-acceptance-testing (macos-latest)	54s
kotlin-acceptance-testing (macos-12)	1m 45s
kotlin-acceptance-testing (windows-latest)	1m 40s
kotlin-deploy-snapshot	1m 47s
	47m 40s

Provide feedback