docs(python-sdk): document retain_async kwarg per #1306

[r266-tech]

Summary

#1306 added retain_async: bool = False to client.retain() / client.aretain() so the convenience wrappers reach parity with retain_batch() / aretain_batch() and the TypeScript SDK. The Python SDK doc still documents the kwarg only on retain_batch() (line 93), leaving the new behavior undiscoverable from ### Retain (Store Memory).

This PR adds the same retain_async=False # Set True for background processing line to the existing "With options" example for client.retain(), mirroring the wording used in the retain_batch block right below it (line 93) and in the Node.js SDK doc's retain() example (async: false).

Changes

• hindsight-docs/docs/sdks/python.md: +1 line in the Retain (Store Memory) "With options" example.
• skills/hindsight-docs/references/sdks/python.md: same +1 line in the skill mirror.

Notes

• Defaults match the implementation in feat(python-client): expose retain_async in retain() and aretain() #1306 (retain_async: bool = False, fully backwards compatible).
• versioned_docs/version-0.5/sdks/python.md mirror is intentionally left for the next chore(docs): sync version-0.5 docs from next pass (cf. chore(docs): sync version-0.5 docs from next #1328).
• No code changes; doctest harness should be unaffected.