Pleroma API Readme: Document unreaction endpoint.

docs/api/pleroma_api.md

@@ -483,6 +483,13 @@ Emoji reactions work a lot like favourites do. They make it possible to react to
 * Params: `emoji`: A single character unicode emoji
 * Response: JSON, the status.
 
+## `POST /api/v1/pleroma/statuses/:id/unreact_with_emoji`
+### Remove a reaction to a post with a unicode emoji
+* Method: `POST`
+* Authentication: required
+* Params: `emoji`: A single character unicode emoji
+* Response: JSON, the status.
+
 ## `GET /api/v1/pleroma/statuses/:id/emoji_reactions_by`
 ### Get an object of emoji to account mappings with accounts that reacted to the post
 * Method: `GET`