Properties doc: about deletion

Took me a short while to understand the "is being deleted" to be
nothing more than a `del` call, so I guess it's worth attempting
to be more explicit.

Author: gracinet

src/py_class/py_class.rs

@@ -176,10 +176,13 @@ get its value and, optionally, to set or delete it.
 
 ### Setter details
 
-* The setter is optional.  If omitted, the attribute will be read-only.
+* The setter is optional.  If omitted, the attribute will be read-only
+  and any setting or deleting attempt will raise `AttributeError`.
 * Unlike Python, the setter method name must be different from the property name.
   The setter method name is used to call the setter from Rust.
-* If value is `None` then the property is being deleted.
+* A `None` value represents that the property is being deleted, for instance
+  with Python `del`.
+  As with Python properties, what should happen is entirely up to the implementation.
 * The value type can be any type that implements `FromPyObject`, or a reference or
   optional reference to any type that implements `RefFromPyObject`.  In the latter
   case, the type of the value is `Option<Option<&impl RefFromPyObject>>`, where

tests/test_class.rs

@@ -1261,6 +1261,10 @@ fn properties() {
     py_run!(py, c, "assert c.match");
     assert!(c.r#match(py).unwrap());
 
+    // Instead of really deleting, our setter sets back to 0
+    py_run!(py, c, "delattr(c, 'prop')");
+    py_run!(py, c, "assert c.prop == 0");
+
     py_run!(py, c, "c.prop_by_ref = 'testing'");
     py_run!(py, c, "assert c.prop_by_ref == 'testing'");