Docs: move WindowManager.fileselect_add descriptions into an example

- Allow for formatting and expanded descriptions.
- Clarify the expected types of each property.

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

@@ -0,0 +1,20 @@
+"""
+This method is used from the operators ``invoke`` callback.
+On success they should return ``{'RUNNING_MODAL'}`` otherwise ``{'CANCELLED'}``.
+
+Accepting the file selector will run the operators ``execute`` callback.
+
+The following properties are supported:
+
+``filepath``: ``bpy.props.StringProperty(subtype='FILE_PATH')``
+   Represents the absolute path to the file.
+``dirpath``: ``bpy.props.StringProperty(subtype='DIR_PATH')``
+   Represents the absolute path to the directory.
+``dirpath``: ``bpy.props.StringProperty(subtype='DIR_PATH')``
+   Represents the filename without the leading directory.
+``files``: ``bpy.props.CollectionProperty(type=bpy.types.OperatorFileListElement)``
+   When present in the operator this collection includes all selected files.
+``filter_glob``: ``bpy.props.StringProperty(default="*.ext")``
+   When present in the operator and it's not empty,
+   it will be used as a file filter (example value: ``*.zip;*.py;*.exe``).
+"""