docs: clarify how to create windows from ActiveEventLoop

[tatakaisun]

Fixes #3877.

Reproduction

The crate-level "Building windows" documentation currently shows creating an
EventLoop, then immediately says to create a Window with create_window.
That skips the step where users obtain an ActiveEventLoop, which is the value
that actually exposes create_window.

Why this is a small fix

The existing example already creates the window from an ApplicationHandler
callback. This PR only adds the missing explanation: EventLoop::run_app()
launches the loop and passes an ActiveEventLoop to the handler callbacks, and
that active event loop should be used to create the window.

Backward compatibility

This is documentation-only. No API, behavior, examples, or runtime code are
changed.

Checks

• cargo check -p winit passed
• cargo test -p winit --doc passed
• cargo fmt --check fails due to existing formatting / rustfmt configuration unrelated to this docs-only change