docs: describe FromDI as a taskiq dependency in README

[lesnik512]

Summary

The README called FromDI an inert marker and said setup_di builds the per-task REQUEST child.

Evidence:

• modern_di_taskiq/main.py FromDI(dependency, *, use_cache=True) returns TaskiqDepends(Dependency(...), use_cache=use_cache).
• setup_di only stores the container, adds taskiq_message_provider, and registers startup/shutdown handlers. The child is built in build_di_container, a sub-dependency of Dependency.__call__.

Changes

• Usage paragraph and setup_di row: the FromDI dependency builds the child; setup_di registers the message provider.
• FromDI row: real signature and behavior.
• Rewrite the unclear async-finalizer sentence plainly; remove dashes.
• Footer: "with IoC container" -> "with an IoC container", and the org-profile line no longer uses a dash.

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

Docs-only change. just lint-ci and just test pass; no mkdocs.yml in this repo.