MassInvPC: update docstring (#4542)

Author: pbrubeck

firedrake/preconditioners/massinv.py

@@ -5,20 +5,25 @@
 
 
 class MassInvPC(AssembledPC):
+    """A matrix free operator that assembles the mass matrix in the provided space.
 
-    _prefix = "Mp_"
-
-    """A matrix free operator that inverts the mass matrix in the provided space.
-
-    Internally this creates a PETSc KSP object that can be controlled
+    Internally this creates a PETSc PC object that can be controlled
     by options using the extra options prefix ``Mp_``.
 
+    A preconditioner with the (approximate) action of the inverse mass matrix
+    can be constructed by creating an internal PETSc KSP object by setting
+    ``-Mp_pc_type ksp`` and suitable inner options, such as
+    ``-Mp_ksp_ksp_type chebyshev`` and ``-Mp_ksp_pc_type jacobi``, for example.
+
     For Stokes problems, to be spectrally equivalent to the Schur
     complement, the mass matrix should be weighted by the viscosity.
     This can be provided (defaulting to constant viscosity) by
     providing a field defining the viscosity in the application
     context, keyed on ``"mu"``.
     """
+
+    _prefix = "Mp_"
+
     def form(self, pc, test, trial):
         _, bcs = super(MassInvPC, self).form(pc)