docs: add note about throwing in cancelAll and destroy() (#5408)

docs/uppy-core.mdx

@@ -704,7 +704,8 @@ uppy.removeFile('uppyteamkongjpg1501851828779');
 #### `clear()`
 
 Clear the state. Can be useful for manually resetting Uppy after a successful
-upload.
+upload. Note that this method might throw an error if you try to call it while
+an upload is ongoing.
 
 Upload plugins may choose to throw an error if called during an upload.
 
@@ -773,7 +774,9 @@ Retry all uploads (after an error, for example).
 
 #### `cancelAll()`
 
-Cancel all uploads, reset progress and remove all files.
+Cancel all uploads, reset progress and remove all files. If you are using the
+Transloadit plugin, this will also cancel all running assemblies, even after an
+upload has finished.
 
 #### `setState(patch)`
 
@@ -880,7 +883,9 @@ uppy.getPlugin('Dashboard').setOptions({
 #### `destroy()`
 
 Uninstall all plugins and close down this Uppy instance. Also runs
-`uppy.cancelAll()` before uninstalling.
+`uppy.cancelAll()` before uninstalling. Note that this method should not
+normally be used. If you only want reset the Uppy instance so that you can start
+a new upload, you probably want to use `clear()` method instead.
 
 #### `logout()`