Fix CalculatResolution regression and update docs.

Refs #10198.

Code/Mantid/docs/source/algorithms/CalculateResolution-v1.rst

@@ -14,17 +14,24 @@ the reflectometry resolution (dq/q) from them. If no value is provided for two t
 then CalculateResolution will attempt to fetch a value from the workspace's log
 using the two theta log name provided.
 
+CalculateResolution outputs two values, the calculated resolution and the value of
+TwoTheta that was used in the calculation. The latter is useful when TwoTheta was not
+given to CalculateResolution, causing CalculateResolution to extract it from the
+workspace's sample log.
+
 Usage
 -----
 
 .. testcode::
 
   ws = Load('INTER00013460')
-  res = CalculateResolution(Workspace = ws, TwoTheta = 0.7)
+  res, two_theta = CalculateResolution(Workspace = ws, TwoTheta = 0.7)
   print("Resolution: %.4f" % res)
+  print("Two Theta: %.4f" % two_theta)
 
 .. testoutput::
 
   Resolution: 0.0340
+  Two Theta: 0.7000
 
 .. categories::