After submitting a tag wiki edit, you can improve your own suggested edit by hitting the Edit Tag Info button:
(sidenote: why does the profanity tag have VB syntax highlighting?!)
However, if you're rate limited (because of the maximum of 5 concurrent pending suggested edits), you can't improve it anymore, it just shows 'edit pending'.
This could be because the edit page also allows you to modify the excerpt (but on the other hand, if you have 4 pending edits, you can modify both the wiki and the excerpt, bringing you at 6).
Also, related: if you use the browser's back button to go back to the edit page, you can't submit the suggested edit because of too many pending suggested edits. This is different from how suggested edits to normal posts work; there, you are able to improve a pending suggested edit even if you have 5 of them pending review.