ZOOKEEPER-5050. Disable AdminServer and enhance documentation to highlight security considerations #3203

Job	Run time
Typo Check	6s
mvn (full-build-jdk11, 11, -Pfull-build apache-rat:check verify -DskipTests spotbugs:check checks...	3m 37s
mvn (full-build-jdk8, 8, -Pfull-build apache-rat:check verify -DskipTests spotbugs:check checksty...	3m 0s
mvn (full-build-java-tests, 11, -Pfull-build verify -Dsurefire-forkcount=1 -DskipCppUnit -Dsurefi...	1h 11m 20s
mvn (full-build-cppunit-tests, 11, -Pfull-build verify -Dtest=_ -DfailIfNoTests=false)	11m 20s
	1h 29m 23s

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