PyDoc: expanded documentation for bpy_struct.is_property_set

This patch expands on the `is_property_set` doc-string,
detailing the purpose of the `ghost` argument added in
d3bcbe10c2.

Reviewed By: sybren

Ref D9780

doc/python_api/examples/bpy.types.bpy_struct.is_property_set.py

@@ -0,0 +1,30 @@
+"""
+.. note::
+
+   Properties defined at run-time store the values of the properties as custom-properties.
+
+   This method checks if the underlying data exists, causing the property to be considered *set*.
+
+   A common pattern for operators is to calculate a value for the properties
+   that have not had their values explicitly set by the caller
+   (where the caller could be a key-binding, menu-items or Python script for example).
+
+   In the case of executing operators multiple times, values are re-used from the previous execution.
+
+   For example: subdividing a mesh with a smooth value of 1.0 will keep using
+   that value on subsequent calls to subdivision, unless the operator is called with
+   that property set to a different value.
+
+   This behavior can be disabled using the ``SKIP_SAVE`` option when the property is declared (see: :mod:`bpy.props`).
+
+   The ``ghost`` argument allows detecting how a value from a previous execution is handled.
+
+   - When true: The property is considered unset even if the value from a previous call is used.
+   - When false: The existence of any values causes ``is_property_set`` to return true.
+
+   While this argument should typically be omitted, there are times when
+   it's important to know if a value is anything besides the default.
+
+   For example, the previous value may have been scaled by the scene's unit scale.
+   In this case scaling the value multiple times would cause problems, so the ``ghost`` argument should be false.
+"""