Clarify implicit_reexport with strict mode

[ahmojo]

Fixes #16971.

Summary

Clarifies that implicit_reexport is enabled by default in normal configuration, but strict enables --no-implicit-reexport unless users explicitly set implicit_reexport = true.

Why

The implementation default is still implicit_reexport = true, but users running with strict = true can observe behavior equivalent to implicit_reexport = false. This keeps the existing behavior and documents the strict-mode interaction near the config option.

Validation

• Local two-file repro: default behavior allows implicit reexport.
• Local two-file repro: --strict reports [attr-defined] for the implicit reexport.
• Local config repro: strict = true reports [attr-defined].
• Local config repro: strict = true plus implicit_reexport = true passes.
• sphinx-build -b html ... against a copied docs source tree succeeded; it emitted existing changelog toctree warnings.