Add ProxySetProps docstring

Author: T4rk1n

dash/long_callback/_proxy_set_props.py

@@ -1,4 +1,9 @@
 class ProxySetProps(dict):
+    """
+    Defer dictionary item setter to run a custom function on change.
+    Used by background callback manager to save the `set_props` data.
+    """
+
     def __init__(self, on_change):
         super().__init__()
         self.on_change = on_change