STYLE: mappedPatchFieldBase: added comment

src/finiteVolume/fields/fvPatchFields/derived/mappedField/mappedPatchFieldBase.H

@@ -25,7 +25,18 @@ Class
     Foam::mappedPatchFieldBase
 
 Description
-    Functionality for sampling fields using mappedPatchBase.
+    Functionality for sampling fields using mappedPatchBase. Every call to
+    mappedField() returns a sampled field, optionally scaled to maintain an
+    area-weighted average.
+
+    Example usage:
+
+    {
+        fieldName           T;          // default is same as fvPatchField
+        setAverage          false;
+        average             1.0;        // only if setAverage=true
+        interpolationScheme cellPoint;  // default is cell
+    }
 
 SourceFiles
     mappedPatchFieldBase.C