pg_checksums docs mentioning online functions

Looking at the pg_checksums docs, it says:

	pg_checksums checks, enables or disables data checksums in a
	PostgreSQL cluster. The server must be shut down cleanly before
	running pg_checksums.

Do we want to mention the new functions here that can change the
checksum status without being shut down?

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  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

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