Fix hypre namespace description

These settings must be preceded by the hypre_namespace setting corresponding 

to the solver (see :ref:`Chap:InputsMultigrid`).

NOTE: If either the :cpp:`nodal_proj` or :cpp:`mac_proj` bottom solvers are 

set to :cpp:`hypre`, the default behavior is to set the :cpp:`hypre_namespace = "hypre"`. 

In that case, the settings below should be preceded by the (default) hypre namespace, 

:cpp:`hypre`, and apply to either or all solvers which use hypre. However, 

the hypre namespace can be set at the solver level so that the hypre settings 

can be configured for each MLMG solver. In which case, the settings below should be 

preceded by the solver specific :cpp:`hypre_namespace`. 

NOTE: By default, the MAC and nodal projections use the same settings 

specified with the namespace :cpp:`hypre`, however the solvers can be configured 

separately by specifying :cpp:`mac_proj.hypre_namespace` and :cpp:`nodal_proj.hypre_namespace`.

When not using the default namespace :cpp:`hypre`, additional restrictions apply:

#. Both :cpp:`mac_proj.hypre_namespace` and :cpp:`nodal_proj.hypre_namespace` must be set.

#. :cpp:`mac_proj.hypre_namespace` and :cpp:`nodal_proj.hypre_namespace` must be unique.   

+-----------------------------------+-----------------------------------------------------------------------+-------------+--------------+