Docs: add warning regarding operators that use the file selector

doc/python_api/examples/bpy.types.WindowManager.fileselect_add.py

@@ -21,4 +21,25 @@ The following properties are supported:
    If this property is present and set to ``True``,
    the operator will warn if the provided file-path already exists
    by highlighting the filename input field in red.
+
+
+.. warning::
+
+   After opening the file-browser the user may continue to use Blender,
+   this means it is possible for the user to change the context in ways
+   that would cause the operators ``poll`` function to fail.
+
+   Unless the operator reads all necessary data from the context before the file-selector is opened,
+   it is recommended for operators to check the ``poll`` function from ``execute``
+   to ensure the context is still valid.
+
+   Example from the body of an operators ``execute`` function:
+
+   .. code-block:: python
+
+      if self.options.is_invoke:
+          # The context may have changed since invoking the file selector.
+          if not self.poll(context):
+              self.report({'ERROR'}, "Invalid context")
+              return {'CANCELLED'}
 """