ZOOKEEPER-4902: Document that read-only mode also enables isro 4lw

[cnauroth]

No description provided.

[cnauroth]

Code reference showing where this is done:

zookeeper/zookeeper-server/src/main/java/org/apache/zookeeper/server/command/FourLetterCommands.java

Lines 217 to 219 in 160297d

	if (System.getProperty("readonlymode.enabled", "false").equals("true")) {
	whiteListedCommands.add("isro");
	}

[cnauroth]

@anmolnar , @eolivelli , @tisonkun , are any of you available for a code review of this small documentation change? Thank you.

[cnauroth]

I have committed this to master, branch-3.9 and branch-3.8. Thank you for the reviews, @tisonkun and @kezhuw .