[docs] Clarify config isn't forwarded with custom onEntrypointLoaded

[StolarczykJakub]

Description of what this PR is changing or adding, and why:

The web initialization docs imply that setting config in
_flutter.loader.load() is enough for it to reach the engine. That's only
true on the default auto-start path. When a custom onEntrypointLoaded
callback is supplied, the loader passes that callback only the engine
initializer — the config from load() is not forwarded, so it is
silently dropped unless it is passed to initializeEngine(config) directly.

The existing "Display a progress indicator" example calls
initializeEngine() with no arguments, which reinforces the
misunderstanding in the most commonly copied onEntrypointLoaded snippet.

Why this matters: the failure is silent and easy to hit in production. A
custom onEntrypointLoaded that sets canvasKitForceCpuOnly: true to work
around a CanvasKit rendering bug in an Android in-app browser has no effect,
and produces no error or warning. The only tell is an absent signal: the
engine always logs Falling back to CPU-only rendering when CPU-only is
active, so the missing log line is the sole hint that the config never
reached the engine. Because this failure mode is completely silent, a doc
note is the only thing that surfaces it.

This PR makes two changes to
platform-integration/web/initialization.md:

• Adds a :::warning in the onEntrypointLoaded callback section stating
  that config from load() is not forwarded when a custom
  onEntrypointLoaded is used, with a short example that passes the config
  to initializeEngine().
• Adds a clarifying comment to the no-argument initializeEngine() call in
  the progress-indicator example.

Verified against the engine's flutter.js loader source: the default
onEntrypointLoaded closure is the only place that captures config and
passes it to initializeEngine(config) (in the minified loader the default
callback is l => l.initializeEngine(t)). When a custom callback is
supplied, didCreateEngineInitializer forwards only the engine
initializer, so the config never reaches the engine automatically. Scope is
a single Markdown file, with no code or config changes.

Issues fixed by this PR (if any):

None.

PRs or commits this PR depends on (if any):

None.

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[gemini-code-assist]

Code Review

This pull request updates the Flutter Web initialization documentation to warn users that configuration is not automatically forwarded to the engine when using a custom onEntrypointLoaded callback. The reviewer suggested clarifying a comment in the example code to make it clear that passing the configuration is conditional, as no configuration is defined in that specific example.