Can't find JIRA.* docs

I started reading the docs ( a few days ago and I see many samples with : JIRA.[something]

For exmaple:

JIRA.Messages.showWarningMsg('sample warning msg')

These samples are cutted part from code, so it doesn’t matters what it does, but they are very similliar to these which I saw in Scriptrunner when I worked with Groovy so they are attractive for me, but I really can’t find complete list/doc with this modules/packages (don’t know how to call this) and don’t want to guess, so anyone knows where can I find it?

My currently problem is check that logged user is in selected group so I hope it is possible to do with this tool

Except that (but it’s not necessery) if someone can recommend other good docs related to developing with atlassian product I will be very happy, because there are so many of them that you can get lost and I don’t know what to focus on :smile:

Sample source:

Thanks in advance