Clarify CertificateParams::signed_by() docs #1718

Job	Run time
Validate external types appearing in public API	1m 40s
build-windows	7m 2s
build (macOS-latest, stable)	32s
Build rustls-cert-gen as a standalone package	32s
build (ubuntu-latest, stable)	38s
Measure coverage	6m 52s
Format & clippy	6m 49s
build (ubuntu-latest, beta)	6m 51s
build (ubuntu-latest, nightly)	7m 3s
Documentation (stable)	1m 6s
build (ubuntu-latest, stable 7 months ago)	42s
Documentation (nightly)	1m 14s
Check semver compatibility	54s
	41m 55s

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