Documentation for JIRA 5.2. Documentation for other versions of JIRA is available too.
As an extension to JIRA, Atlassian have developed a set of JIRA custom fields, collectively called the "JIRA Toolkit". It can be found online at the Atlassian Plugin Exchange
These custom fields are particularly useful in customer-facing JIRA instances. They were initially developed for use in Atlassian's own JIRA Support installation at http://support.atlassian.com. See the JIRA Toolkit documentation for details.