message: Update documentation for preconditioner's SetOperator function
diff: @@ -511,7 +511,8 @@ public:
  virtual ~GinkgoPreconditioner() = default;
  /**
  - * Generate the preconditioner for the given matrix @p op.
  + * Generate the preconditioner for the given matrix @p op,
  + * which must be of MFEM SparseMatrix type.
  * Calling this function is only required when creating a
  * preconditioner for use with another MFEM solver; to use with
  * a Ginkgo solver, get the LinOpFactory pointer through @p GetFactory()
  