Clarify CertificateParams::signed_by() docs #1717

Job	Run time
Validate external types appearing in public API	1m 42s
Build rustls-cert-gen as a standalone package	27s
build (macOS-latest, stable)	34s
build-windows	8m 0s
build (ubuntu-latest, stable)	33s
Measure coverage	6m 37s
Format & clippy	6m 24s
build (ubuntu-latest, beta)	6m 39s
Documentation (stable)	1m 35s
build (ubuntu-latest, nightly)	6m 43s
Check semver compatibility	58s
build (ubuntu-latest, stable 7 months ago)	40s
Documentation (nightly)	1m 13s
	42m 5s

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