r.univar: add note that parallelization is disabled if mask is set (#…

…3562)

And remove older note about extended statistics that is not valid anymore.

raster/r.univar/r.univar.html

@@ -53,8 +53,7 @@ <h3>PERFORMANCE</h3>
 <p>
 <em>r.univar</em> supports parallel processing using OpenMP. The user
 can specify the number of threads to be used with the <b>nprocs</b> parameter.
-However, parallelization is disabled when the <b>-e</b> extended statistics
-flag is used.
+However, parallelization is disabled when the MASK is set.
 
 <p>
 Due to the differences in summation order, users may encounter small floating points