ENH: snappyHexMeshDict: added missing documentation

etc/caseDicts/annotated/snappyHexMeshDict

@@ -29,6 +29,11 @@ addLayers       false;
 //           refinement/unrefinement
 // mergePatchFaces    false; // default true
 
+// Optional: allow patch-face merging even if differing patches. This avoids
+// splitting boundary faces and might benefit layer coverage.
+// (Note: only applicable if mergePatchFaces true)
+// mergeAcrossPatches   true;   // default false
+
 // Optional: preserve all generated patches. Default is to remove
 //          zero-sized patches.
 // keepPatches true;
@@ -214,6 +219,13 @@ castellatedMeshControls
             //      boundary : create free-standing boundary faces (baffles
             //                 but without the shared points)
             //faceType baffle;
+
+            //- Optional automatic gap refinement. See section in
+            //  refinementRegions ('gapLevel', 'gapMode')
+
+            //- Optional removal of cells in thin gaps. Start removing cells
+            //  if at refinement level 10 we're detecting thin gaps.
+            //blockLevel 10;
         }
     }