Add flip processing option docs

Author: DarthSim

docs/usage/processing.mdx

@@ -389,6 +389,20 @@ Only 0, 90, 180, 270, etc., degree angles are supported.
 
 Default: 0
 
+### Flip
+
+```imgproxy_url_option
+flip:%horizontal:%vertical
+fl:%horizontal:%vertical
+```
+
+When set, imgproxy will flip the image along the specified axes.
+
+* `horizontal` - _(optional)_ when set to `1`, `t` or `true`, imgproxy will flip the image horizontally.
+* `vertical` - _(optional)_ when set to `1`, `t` or `true`, imgproxy will flip the image vertically.
+
+Default: `false:false`
+
 ### Background
 
 ```imgproxy_url_option