Add note in gen_to_sage pointing out .sage()

[user202729]

As in the title. I feel it's not discoverable enough as is.

(actually the current last example already use .sage(), but that looks too hidden.)

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• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
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[user202729]

Failures are strange, but looks unrelated to the change. (It's not the typical signal issue.)

2025-01-01T11:34:57.3444140Z sage -t --warn-long 5.0 --random-seed=42531168360341585061458755337891210050 src/doc/en/thematic_tutorials/group_theory.rst  # Timed out
2025-01-01T11:34:57.3444585Z sage -t --warn-long 5.0 --random-seed=42531168360341585061458755337891210050 src/sage/combinat/designs/incidence_structures.py  # Timed out
2025-01-01T11:34:57.3444900Z sage -t --warn-long 5.0 --random-seed=42531168360341585061458755337891210050 src/sage/graphs/generic_graph.py  # Timed out
2025-01-01T11:34:57.3445244Z sage -t --warn-long 5.0 --random-seed=42531168360341585061458755337891210050 src/sage/groups/finitely_presented_named.py  # Timed out
2025-01-01T11:34:57.3445552Z sage -t --warn-long 5.0 --random-seed=42531168360341585061458755337891210050 src/sage/groups/libgap_wrapper.pyx  # Timed out
2025-01-01T11:34:57.3445877Z sage -t --warn-long 5.0 --random-seed=42531168360341585061458755337891210050 src/sage/groups/perm_gps/permgroup.py  # Timed out
2025-01-01T11:34:57.3446224Z sage -t --warn-long 5.0 --random-seed=42531168360341585061458755337891210050 src/sage/groups/perm_gps/permgroup_morphism.py  # Timed out
2025-01-01T11:34:57.3446591Z sage -t --warn-long 5.0 --random-seed=42531168360341585061458755337891210050 src/sage/topology/simplicial_complex_examples.py  # Timed out
2025-01-01T11:34:57.3446920Z sage -t --warn-long 5.0 --random-seed=42531168360341585061458755337891210050 src/sage/topology/simplicial_complex.py  # Timed out