[ZEPPELIN-3271] add shiro setup for cron feature in docs

[sh1nj1]

What is this PR for?

This PR adds description of another condition to enable Cron feature in documentation.
It is hard to know that is is required to configure Shiro authentication to enable Cron feature.

What type of PR is it?

Documentation

Todos

What is the Jira issue?

• [ZEPPELIN-3271]

How should this be tested?

• bundle exec jekyll serve --watch and check the documentation updated.

Screenshots (if appropriate)

Questions:

• Does the license files need to update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No

[Reamer]

You are right.

zeppelin/zeppelin-interpreter/src/main/java/org/apache/zeppelin/conf/ZeppelinConfiguration.java

Lines 801 to 803 in 571e8d5

	public boolean isZeppelinNotebookCronEnable() {
	return getBoolean(ConfVars.ZEPPELIN_NOTEBOOK_CRON_ENABLE) && isAuthenticationEnabled();
	}