docs: rewrite Flask setup paragraph plainly

[lesnik512]

Summary

Rewrite the setup paragraph and example comment in plain terms.

Changes

• The paragraph after the example repeated "no shutdown hook" twice, used bold phrases, and carried version history ("As of modern-di 3.1", "Validation is explicit in 3.1"). Split it into an auto_inject paragraph and a short lifecycle paragraph: the container is open from construction, call .validate() after setup_di (which registers flask_request_provider, modern_di_flask/main.py:39), and call fetch_di_container(app).close_sync() at process shutdown.
• The example comment talked about auto_inject, which the example does not use, and had an em dash. Removed it; the validate() comment now matches the other integration READMEs.
• Removed the remaining em dashes in the usage intro and the setup_di API row.
• Footer: "with IoC container" -> "with an IoC container"; replaced the em dash in the org link line.

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered (docs only, no behavior change)
• 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