As we all know, only 0.3% of users read manuals or remember what they are told. Therefore, you need to give contextual help in order to avoid constant support phone call hassles or people simply refusing to use the system because it is “too complicated” or “not intuitive”.
For example, if you may have some tricky requirement such as ‘Security Level must be set to something other than “None” in order for the workflow action “Submit” to appear’.
Luckily there is an excellent plugin that allows you to show arbitrary HTML on Issue Create and Issue View screens: JIRA Toolkit (see Message Custom Fields). When a user is creating an issue you can give them huge warnings in big red letters (with pictures and flashing text) if you want. And after the issue is created, on the View screen you can show other reminders and help messages. When all your users are familiar with the system, you can simply remove the custom fields showing the arbitrary HTML.
Here is an example: