update loudspeaker layout documentation

docs/loudspeaker_layouts.rst

@@ -6,7 +6,19 @@ Loudspeaker Layouts
 An output loudspeaker layout is represented by :class:`Layout`, which contains
 a name, a list of :class:`Channel` objects, and the reference :type:`Screen`.
 
-Although :class:`Layout` objects could be constructed directly from scratch,
-only layouts listed in :cite:`bs2051` are supported. The function
-:func:`getLayout` is therefore provided to obtain a :class:`Layout` object
-given the layout name, e.g. ``4+5+0``.
+Loudspeaker layouts specified in :cite:`bs2051` are supported, including
+positions within the ranges specified. The function :func:`getLayout` is
+therefore provided to obtain a :class:`Layout` object given the layout name,
+e.g. ``4+5+0``.
+
+When using layouts with non-nominal positions, the
+:func:`Channel::polarPositionNominal` must match the position specified in Table
+1 in :cite:`bs2051`, and the :func:`Channel::polarPosition` must meet the
+specified constraints, including azimuth/elevation ranges and symmetry
+requirements.
+
+Non-standard loudspeaker layouts may be used, however their behaviour may
+change in future versions of the library. Loudspeaker layouts must be similar
+to those in :cite:`bs2051`, with 1, 2 or 3 layers and an optional T+000 or
+UH+180 loudspeaker. Using this functionality requires some understanding of the
+internals of the renderer, particularly section 6.1.3.1 of :cite:`bs2127`.