docs: fix README facts and examples

[lesnik512]

Summary

README fixes for facts that did not match the code, an example that did not run, and some prose cleanup. No code changes; pyproject description untouched.

Changes

• Retry example: class User(DeclarativeBase) raises InvalidRequestError: Cannot use 'DeclarativeBase' directly. Added class Base(DeclarativeBase) and class User(Base).
• DB_RETRY_RETRIES_NUMBER and retries=N were described as the number of retries. db_retry/retry.py:46 uses tenacity.stop_after_attempt(N), which counts the first call, so the default 3 means 2 retries (checked: a function always raising SerializationError is called 3 times). Now worded as total attempts.
• Transaction was described as only rolling back. db_retry/transaction.py:22-24 rolls back if still in a transaction, then always closes the session.
• "Engine will automatically try different hosts on failure" overstated failover. db_retry/connections.py:88 falls back host by host only when the first multi-host connect raises TimeoutError; other errors propagate.
• Multi-host example imported sa, AsyncSession, DeclarativeBase, mapped_column without using them; removed.
• build_db_dsn also sets target_session_attrs to read-write, or prefer-standby with use_replica=True (db_retry/dsn.py:26); documented.
• Retry intro now names the retried errors: asyncpg SerializationError and PostgresConnectionError (db_retry/retriable.py:5).
• Sentence-case headings, plain feature bullets, shorter intro, removed filler words.

Verification: byte-compiled every python block in the README; ran the retry, multi-host and serializable examples against a throwaway Postgres container (all succeed). The transaction example uses placeholder service modules and was only compiled.

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