Add reset-progress event to docs (#1922)

* document reset-progress event

* reword progress reset behaviour

* remove totalProgress reference

packages/@uppy/core/src/index.js

@@ -315,7 +315,6 @@ class Uppy {
       totalProgress: 0
     })
 
-    // TODO Document on the website
     this.emit('reset-progress')
   }
 

website/src/docs/uppy.md

@@ -755,3 +755,13 @@ uppy.on('restriction-failed', (file, error) => {
   // do some customized logic like showing system notice to users
 })
 ```
+
+### `reset-progress`
+
+Fired when `uppy.resetProgress()` is called, each file has its upload progress reset to zero.
+
+```javascript
+uppy.on('reset-progress', () => {
+  // progress was reset
+})
+```