refs #9955. Document and format.

Code/Mantid/docs/source/algorithms/Stitch1D-v3.rst

@@ -22,6 +22,16 @@ The workspaces must be histogrammed. Use
 :ref:`algm-ConvertToHistogram` on workspaces prior to
 passing them to this algorithm.
 
+**Special Value Processing**
+
+Special values, meaning infinite and NaN double precision values, are treated in a different way by the algorithm. The processing is as follows:
+
+#. The workspaces are initially rebinned, as prescribed by the rebin Params (determined automatically if not provided)
+#. Each spectra is searched for signal values and error values that are special values. Positions and type of special value are recorded
+#. Special values found in the previous step are set to zero
+#. Stitching proceeds, integration will not be performed considering any special values
+#. Post processing the indexes of the special values are used to put the special values back where they came from
+
 Usage
 -----
 **Example - a basic example using stitch1D to stitch two workspaces together.**