Formulate doc-string more precisely.

src/grid.h

@@ -54,7 +54,7 @@ struct gridVirtTable
 #endif
   double (*inqXInc)(grid_t *gridptr);
   double (*inqYInc)(grid_t *gridptr);
-  // return if for both grids, all xval and all yval are equal
+  // return true if for both grids, any xval and all yval differ
   bool (*compareXYFull)(grid_t *gridRef, grid_t *gridTest);
   // return if for both grids, x[0], y[0], x[size-1] and y[size-1] are respectively equal
   bool (*compareXYAO)(grid_t *gridRef, grid_t *gridTest);