common-ekf-sources: Add comments around source options

common/source/docs/common-ekf-sources.rst

@@ -27,4 +27,10 @@ Three set of EKF position and velocity source parameters are provided which can
 
 This feature is especially helpful when using :ref:`common-non-gps-to-gps`.
 
+Source Switching
+================
+
+There is an option to fuse all sources of velocity into the EKF.  This will take velocity information from the XY and Z sources defined by :ref:`EK3_SRC1_VELXY<EK3_SRCx_VELXY>` and :ref:`EK3_SRC1_VELZ<EK3_SRCx_VELZ>`, where 'x' is the source number (1-3).  To enable this option set the first bit in the :ref:`EK3_SRC_OPTIONS<EK3_SRC_OPTIONS>` bitmask.
+
+.. warning:: When using this option, users should ensure that the velocity measurements being provided to the EKF are both in the same reference frame/co-ordinate system.