Documentation: Update Docs for Gizmo

This patch updates the documentation for arguments regarding the `Gizmo`
type.

- Corrected `select_id` doc for draw_preset_ functions. `-1` indicates
  that no selection ID is to be written, but previous docs incorrectly
  specified `0` instead.
- Added missing doc for `target` argument for `target_set_handler`
  function.

Reviewed by: Aaron Carlisle (Blendify)

Differential Revision: https://developer.blender.org/D14834

source/blender/python/intern/bpy_rna_gizmo.c

@@ -314,6 +314,8 @@ PyDoc_STRVAR(
     "\n"
     "   Assigns callbacks to a gizmos property.\n"
     "\n"
+    "   :arg target: Target property name.\n"
+    "   :type target: string\n"
     "   :arg get: Function that returns the value for this property (single value or sequence).\n"
     "   :type get: callable\n"
     "   :arg set: Function that takes a single value argument and applies it.\n"