docs: clarify query lifecycle timing

[MuslumYilmaz]

Summary

• clarify that the example obtains Apollo during construction but starts the query in ngOnInit
• explain how to update input-derived query variables after initialization

Verification

• Reviewed the rendered Markdown preview
• Cross-checked the lifecycle timing and current QueryRef.setVariables() API
• Full website build (not run in this browser-only documentation workflow)

Checklist

• This is a small documentation clarification; no feature-design discussion is required.
• No runtime logic was added, so no runtime test or changeset is needed.

Disclosure

I maintain FrontendAtlas, the optional external resource linked once in the added note. The Apollo-specific guidance stands on its own without that link, and I am happy to remove it if the project prefers first-party-only references.

This contribution was prepared with AI assistance. I reviewed and validated the final wording and technical claims before submission.