Documentation is what helps every Joomla site administrator to learn their Joomla system and installed Joomla extensions.

Joomla system is well documented. You can find a lot of useful information at docs.joomla.org.

joomla documentaion

You can also find documentation for an extension purchased here, at Joomlashack. Simply go to the "Joomla Extensions" page and click on the name of the extension. 

You will be taken to the extension's page. Find the "Key Links" menu and click "Documentation":

click documentation

You will be taken to the extension's documentation page:

jcal pro documentation