Clarify CSR signing docs #1589

Job	Run time
Build rustls-cert-gen as a standalone package	35s
build (macOS-latest, stable)	5m 30s
build-windows	11m 7s
Validate external types appearing in public API	1m 31s
build (ubuntu-latest, stable)	6m 3s
build (ubuntu-latest, beta)	5m 51s
Measure coverage	5m 48s
build (ubuntu-latest, nightly)	5m 44s
Format & clippy	5m 22s
build (ubuntu-latest, stable 7 months ago)	5m 58s
Documentation (stable)	57s
Check semver compatibility	57s
Documentation (nightly)	1m 6s
	56m 29s

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