update documentation for PETSc PreconditionLU

[tjhei]

No description provided.

[drwells]

I think that this also works with PETScWrappers::MPI::SparseMatrix as long as we run on a single processor. Let me check.

[drwells]

Yes: this preconditioner works as long as the matrix only lives on a single processor. Since we will deprecate PETScWrappers::SparseMatrix at some point soon (like we did for the non-parallel vector classes) I would prefer that we mention this.

[bangerth]

Oh, I guess I pre-empted a discussion here. My apologies.

[drwells]

Its not a problem: this is a pretty obscure use case for the parallel matrix.

When I have a bit more time I'll clean up the PETSc wrappers some more, which will involve deprecating this class anyway.