Improve algorithm documentation

docs/source/algorithms/ExportGeometry-v1.rst

@@ -11,8 +11,11 @@ Description
 -----------
 
 This algorithm is intended to write out portions of an instrument's
-geometry. The resulting file is mean to be copied by-hand into a
-geometry file. The main use of this is if the instrument geometry is
+geometry in the instrument definition file xml format. The resulting
+file is meant to be copied by-hand into a geometry file, the output
+is not a useable IDF as written. 
+
+The main use of this algorithm is if the instrument geometry is
 calibrated in mantid, this algorithm can be used to help get the
 information back into the initial instrument definition file.