add choices to documentaton for paramater flat (#43939)

lib/ansible/modules/files/fetch.py

@@ -57,6 +57,8 @@ options:
         hostname/path/to/file to the destination.  If dest ends with '/', it
         will use the basename of the source file, similar to the copy module.
         Obviously this is only handy if the filenames are unique.
+    type: bool
+    default: 'no'
 author:
     - "Ansible Core Team"
     - "Michael DeHaan"