docs(core): tighten Python group rationale to reflect measured shape

The initial comment implied the file-pattern rules matter for pandas /
numpy / scikit-learn / tensorflow uniformly. Measurement across 17
public Python repos showed the picture is bimodal: cluster-in-tests/
projects (django, flask, pandas, numpy, scikit-learn) see near-zero
extra savings because existing directory rules already exclude those
paths, while Google-style interleaved projects (tensorflow at 47%,
plus jax / pytorch / cpython lower down) account for essentially all
the token reduction.

Update the block comment so a future reader lands on the right mental
model when deciding whether to activate these suggestions.