Currently, it seems that Priorities Description field in the Priorities page does not use the Wiki Style Renderer like the other Fields in JIRA. User able to use the style only on the description field in the Field Configuration page.

This feature would be good if the priority description is quite long, it would not written in one line but rather in multiple line or could give a color in the text to emphasis it. In general, to make the description more better than one piece of a line.