Clarify CertificateParams::signed_by() docs #1721

Job	Run time
Check semver compatibility	56s
Documentation (stable)	1m 10s
Documentation (nightly)	1m 11s
Build rustls-cert-gen as a standalone package	23s
build (macOS-latest, stable)	33s
build-windows	7m 28s
build (ubuntu-latest, stable)	33s
Validate external types appearing in public API	1m 41s
build (ubuntu-latest, beta)	6m 46s
Format & clippy	6m 27s
build (ubuntu-latest, nightly)	6m 37s
Measure coverage	6m 50s
build (ubuntu-latest, stable 7 months ago)	50s
	41m 25s

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