Re #11863 add documentation

Code/Mantid/docs/source/algorithms/EQSANSDarkCurrentSubtraction-v2.rst

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+.. algorithm::
+
+.. summary::
+
+.. alias::
+
+.. properties::
+
+Description
+-----------
+
+Subtract the dark current from an EQSANS data set. 
+This algorithm is rarely called directly. It is called by 
+:ref:`SANSReduction <algm-SANSReduction>`.
+
+This workflow algorithm will:
+
+- Properly load the dark current data set
+
+- Normalize the dark current to the data taking period
+
+- Subtract the dark current from the input workspace
+
+
+The dark current is subtracted pixel by pixel by normalizing the dark current data by counting time. The total number of dark current counts :math:`N_{dc}(i)` for each pixel i is obtained by integrating over all time of flight bins. For a given pixel and wavelength bin, the corrected signal is given by:
+
+:math:`I'(i,\lambda_j)=I_{data}(i,\lambda_j)-N_{dc}(i) \ \ \frac{T_{data}}{T_{dc}} \ \ \frac{t_{frame}-t^{low}_{cut} - t^{high}_{cut}}{t_{frame}} \ \ \frac{\Delta\lambda_j}{\lambda_{max}-\lambda_{min}}`
+
+where the T-values are the counting times for the data set and the dark current (dc). The :math:`t_{cut}` values are the TOF cuts at the beginning and end of a frame. :math:`t_{frame}` is the width of a frame. :math:`\Delta\lambda_j` is the width of the wavelength bin we are considering. 
+
+.. categories::
+
+.. sourcelink::