docs: separate the mounted and side-by-side container sharing cases

[lesnik512]

The setup_di row said one app should own the lifespan and the others pass manage_lifespan=False. That holds when the FastMCP server is mounted in a FastAPI app whose lifespan runs it, but read as general advice.

The row now covers both cases, matching the modern-di docs:

• Mounted in another app whose lifespan runs it: that app owns the container, FastMCP passes manage_lifespan=False. Links the FastMCP integration docs section.
• Separate lifecycles: pass manage_lifespan=False to every setup_di and open and close the container yourself.