Generic RICH documentation

Documented the generic RICH detector.

docs/detectors.rst

@@ -15,9 +15,31 @@ Particle Identification (PID)
 Generic Gaseous RICH
 ====================
 
+A generic gas RICH detector for hadron PID in the forward dectection is shown in the figure below.
+The simple design uses a plane mirror to reflect cherenkov photons (green) onto an array of MCP-PMT detectors (blue).
+The optical properties of the mirror are defined in  a properites block as `shown here <https://eicweb.phy.anl.gov/EIC/NPDet/-/blob/master/src/GenericDetectors/pid/compact/GenericRICH_example.xml#L23>`_.
 
 .. image:: images/topside_rich.png
    :width: 600
 
+.. code-block:: xml
+
+    <detector id="1" name="ForwardRICH" type="GenericRICH" 
+              readout="ForwardRICHHits" vis="RedVis" material="N2cherenkov">
+      <dimensions rmin1="30*cm" rmin2="30*cm" rmax1="80*cm" rmax2="80*cm" zmin="20*cm"  zmax="120*cm"/>
+    </detector>
+
+Optical surfaces are `defined in the detector 
+construction. <https://eicweb.phy.anl.gov/EIC/NPDet/-/blob/master/src/GenericDetectors/pid/src/GenericRICH_geo.cpp#L129>`_
+Their optical properites of surfaces are defined in the detector description: 
+
+.. code-block:: xml
+
+    <surfaces>
+      <opticalsurface finish="polished" model="glisur" name="MirrorOpticalSurface" type="dielectric_metal" value="0">
+        <property name="REFLECTIVITY" ref="REFLECTIVITY_mirror"/>
+        <property name="RINDEX"       coldim="2" values="1.034*eV  1.5   4.136*eV  1.5"/>
+      </opticalsurface>
+    </surfaces>