Currently have to edit velocity templates weight (or not use the built in Description and make a new custom multi line field Descriptions)
JRA-38723 is about an easy ui for velocity view issue screen arrangement similar to the dashboard editor ui
If you place the description on screens above all other fields in your sort order it would go above details, if it's above say a custom multi line text field, that custom field would move out of details and beneath description.
Filed against JIRA 6.2.6
Painful method of editing locations https://answers.atlassian.com/questions/48974/how-do-i-move-the-attachment-and-linked-issue-fields-above-the-description-field-in-jira-5-0-1
<web-panel key="details-module" location="atl.jira.view.issue.left.context" weight="100">
<web-panel key="descriptionmodule" location="atl.jira.view.issue.left.context" weight="200">
Another approach is to have the ability to move multi-line custom fields to the description area - this would enforce better usage of the built-in description (vs just hiding description)