docs(state): correct create_state error contract

[1cbyc]

Summary

• replace the incorrect StateDataError claim on create_state()
• document that configured backend exceptions propagate unchanged

This keeps the documented API contract aligned with the existing non-wrapping implementation.

Fixes #88

Validation

• uv run zensical build --strict
• uv run pytest --cov=fastapi_cachex --cov-report=term-missing — 520 passed, 146 skipped, 91.89% coverage
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy fastapi_cachex --strict
• uv run mypy tests
• uv run mypy scripts
• uv build

Co-authored-by: insisong emmanuelisaacnsisong@gmail.com

[1cbyc]

Upstream commit 180314e independently corrected the same create_state() exception contract, so this PR is now superseded. Closing to avoid duplicating the fix.