Add docs on key rotation #2465

Job	Run time
PHPUnit tests (nightly) (locked, 8.4, ubuntu-latest)	43s
PHPUnit tests (lowest, 8.1, ubuntu-latest)	12s
PHPUnit tests (lowest, 8.2, ubuntu-latest)	18s
PHPUnit tests (lowest, 8.3, ubuntu-latest)	38s
PHPUnit tests (highest, 8.1, ubuntu-latest)	17s
PHPUnit tests (highest, 8.2, ubuntu-latest)	19s
PHPUnit tests (highest, 8.3, ubuntu-latest)	37s
PHPUnit tests (locked, 8.1, ubuntu-latest)	20s
PHPUnit tests (locked, 8.2, ubuntu-latest)	11s
PHPUnit tests (locked, 8.3, ubuntu-latest)	32s
PHPUnit tests (development, 8.1, ubuntu-latest)	17s
PHPUnit tests (development, 8.2, ubuntu-latest)	19s
PHPUnit tests (development, 8.3, ubuntu-latest)	45s
	5m 28s

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