Clarify CertificateParams::signed_by() docs #1719

Job	Run time
build-windows	7m 48s
Build rustls-cert-gen as a standalone package	24s
build (macOS-latest, stable)	39s
build (ubuntu-latest, stable)	34s
Validate external types appearing in public API	1m 41s
Measure coverage	6m 41s
Format & clippy	6m 40s
build (ubuntu-latest, beta)	7m 0s
build (ubuntu-latest, nightly)	6m 43s
Documentation (stable)	1m 5s
Check semver compatibility	53s
build (ubuntu-latest, stable 7 months ago)	42s
Documentation (nightly)	1m 13s
	42m 3s

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