[code:petsc-gmres] Update source comments for stokes2 matrix

Replace stale audikw_1 / MatrixMarket / .petscbin references in the
top-of-file comment and usage string with stokes2 / FreeFEM ObjectView /
stokes2.dat, matching the current README and run.sh behavior.

Signed-off-by: William Dawson <william.dawson@riken.jp>

Author: william-dawson

programs/petsc-gmres/src/GMRES-PETSc.cpp

@@ -1,16 +1,16 @@
 // CSR data transfered to PETSc  Copyright,  Atsushi Suzuki 30 Jul.2025
 //
 // A PETSc KSP (GMRES + GAMG algebraic multigrid preconditioner) benchmark:
-// load a large sparse SPD matrix, solve Ax = b for a known x, report the
-// relative L2 error and solve wall-time.
+// load the stokes2 sparse matrix (a Stokes flow saddle-point system,
+// 4.26M rows, generated via Gmsh/FreeFEM by Atsushi Suzuki), solve Ax = b
+// for a known x, report the relative L2 error and GMRES-iterate wall-time.
 //
-// Loads a pre-converted PETSc binary matrix via MatLoad() against
-// PETSC_COMM_WORLD, so rows are properly partitioned across MPI ranks
-// (memory per rank scales as O(1/ranks), as a real distributed benchmark
-// should). The binary is produced once, offline, from the MatrixMarket
-// form of the benchmark matrix (audikw_1, SuiteSparse Matrix Collection,
-// GHS_psdef group) -- see this app's README.md for where each target
-// system's copy lives and how it was produced.
+// The matrix is written directly to PETSc binary format via FreeFEM's
+// ObjectView (= MatView), so MatLoad() reads it without a conversion step.
+// Rows are partitioned across MPI ranks via MatLoad against
+// PETSC_COMM_WORLD (memory per rank scales as O(1/ranks), as a real
+// distributed benchmark should). See this app's README.md for where each
+// target system's copy of stokes2.dat lives and how it was generated.
 static char help[] = "load a PETSc binary matrix and call the KSP solver\n";
 
 #include <petscksp.h>
@@ -60,7 +60,7 @@ int main(int argc, char **args)
   PetscCall(PetscOptionsGetString(NULL, NULL, "-f", fname, sizeof(fname), &flg));
   if (!flg) {
     PetscCall(PetscPrintf(PETSC_COMM_WORLD,
-			  "usage: %s -f <matrix.petscbin> [-ksp_monitor] [-pc_type gamg] [-log_view]\n",
+			  "usage: %s -f <stokes2.dat> [-ksp_monitor] [-pc_type gamg] [-log_view]\n",
 			  args[0]));
     PetscCall(PetscFinalize());
     exit(-1);