I suppose you are on a server instance. In Cloud, this is already available:
Screen Shot 2016-12-27 at 11.53.39.png
By hovering over the Sprint in backlog view, an edit sprint link appears which opens up the dialog above. There is an issue to add this functionality to server as well, but no news about dates yet. If you want to keep informed, you can watch the following issue: https://jira.atlassian.com/browse/JSW-7308
there is no option in fact in JIRA Server hosting to add Sprint Goal with more characters and additional features. We have released an add on which address this issue and is available for JIRA Server, it has Show/Hide feature (so you can decide to keep whole description above the backlog(s), ability to add attachments and WYSIWYG text editor to highlight most important areas of the sprint goals the team wants to achieve and deliver. Feel free to check this out in the Agile Sprint Goals marketplace profile
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