docs: clarify mixed-device source version requirement

[LauraGPT]

Summary

Clarify the availability of the newly merged #3706 mixed-device placement contract directly beside the English and Chinese Python API examples.

• PyPI 1.4.15 does not contain the fix; link the source merge 4d8e087 and existing source-install instructions.
• Remind readers to record the checkout commit and imported funasr.__file__, because the current development checkout still reports 1.4.15.
• No runtime, example, dependency, test, workflow, or release changes. This does not claim a new PyPI release or original M2 performance validation.

Verification

• Current PyPI metadata: 1.4.15 artifacts uploaded September 9; fix: honor submodel device placement across inference calls #3706 merged September 16.
• Existing Python API and installation documentation contracts: 47 passed.
• Sphinx test module: 1 skipped, because Sphinx is not installed in the existing local docs environment; the repository CI performs the Sphinx/build checks.
• Both full pages rendered through GitHub Markdown; availability notes and PR/commit/install links verified.
• git diff --check: passed.
• Independent read-only review: no P1/P2 findings; reviewer did not rerun tests.