docs: fix README statements that disagree with the code

[lesnik512]

Summary

The README described some behavior differently from what the code does. README-only change.

Changes

• Quick Start said the middleware "enforces" AckPolicy.MANUAL at runtime. _classify in middleware.py (~line 171) returns _PassThrough() for any non-MANUAL policy, as the next README paragraph already says. Now: required for concurrent processing; other policies pass through untouched.
• commit_batch_size was documented as "Max messages per commit batch" and the example comment said "completed tasks". _commit_scheduler.py:73 commits when pending_len >= batch_size, where pending_len counts absorbed tasks (_pending_state.py:128-133), so it is a trigger threshold on pending tasks and a commit can include more.
• How It Works referred to flush_timeout_sec; the public kwarg is rebalance_flush_timeout_sec (middleware.py:262).
• KafkaConcurrentHandler and KafkaBatchCommitter were presented as core concepts but are not exported from __init__.py; the section now says they are internal.
• Removed version-history wording ("Before 0.6.4...") and the "Migration from < 0.x" section. Its try/finally on CancelledError advice moved under Shutdown.
• Plainer wording for the signal-handler feature bullet and the direct ack()/nack()/reject() limitation; removed a redundant closing sentence in Other limitations.

Both README Python blocks byte-compile; the Quick Start ran end to end under TestKafkaBroker (with KafkaBroker(...) replaced by KafkaBroker()).

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging