Updated to include description of of where x,y,z coordinates for inserting an antenna model refer to.

docs/source/user_libs_antennas.rst

@@ -29,7 +29,7 @@ Module overview
 How to use the module
 =====================
 
-The antenna models can be accessed from within a block of Python code in an input file. The models must be used with cubic spatial resolutions of either 1mm (default) or 2mm by setting the keyword argument, e.g. ``resolution=0.002``. The antenna models can be rotated 90 degrees counter-clockwise (CCW) in the x-y plane by setting the keyword argument ``rotate90=True``.
+The antenna models can be accessed from within a block of Python code in an input file. The models are inserted at location x,y,z. The coordinates are relative to the geometric centre of the antenna in the x-y plane and the bottom of the antenna skid in the z direction. The models must be used with cubic spatial resolutions of either 1mm (default) or 2mm by setting the keyword argument, e.g. ``resolution=0.002``. The antenna models can be rotated 90 degrees counter-clockwise (CCW) in the x-y plane by setting the keyword argument ``rotate90=True``.
 
 Example
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