Add small documentation for the load_balance: (Vh) -> (Vh) function

doc/userdoc.org

@@ -4495,6 +4495,21 @@ to different meshes.
 
 A good example of the use of this kind of function is mass fractions.
 
+***** ~load_balance: (Vh) -> (Vh)~
+
+This function performs a parallel load balancing of a list of ~Vh~
+variables. All the input variables must be defined on a same mesh. The
+return list consists is the balance variables using the input
+ordering. The new (balanced) mesh is carried out by the new variables
+and can be retrieved using the ~get_mesh: Vh -> mesh~ function.
+
+#+BEGIN_warning
+Using this function in a sequential calculation has the same behavior:
+the discrete functions, the mesh and its connectivity are copied. This
+is intended to simplify development of large calculations on simpler
+tests, and for consistency.
+#+END_warning
+
 ***** Numerical methods
 
 We describe rapidly two functions that embed numerical methods. These