docs: clarify xcconfig file usage and simplify notes in iOS flavors guide

[lamek]

Clarifies the iOS flavors guide to explain that both Profile and Release build configurations use Release.xcconfig by default, resolving confusion about a non-existent Profile.xcconfig file.

Moved both notes (existing note and my new one) to inline instructions. This is to avoid stacked callouts at the end of the procedure, or mid-list callouts.

Fixes #12225

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[gemini-code-assist]

Code Review

This pull request updates the iOS Flutter flavors documentation to improve readability, refine phrasing, and clarify the default behavior of Xcode build configurations. The review feedback suggests reverting the relocation of the lowercase scheme name requirement note, as placing it inline under the staging configuration incorrectly implies it only applies to staging. Instead, the reviewer recommends combining it with the general configuration note block to ensure it remains applicable to all configurations.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 9bbb8eb):

https://flutter-docs-prod--docs-pr13748-ticket-542694934-issue-5pskfpz6.web.app

[lamek]

@MuthuGCodes please take a look and ping @sfshaza2 after you've had a chance to review.

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 9bbb8eb):

https://flutter-dev-230821--www-pr13748-ticket-542694934-issue-7xntudor.web.app

[sfshaza2]

I'll approve, but please stop using "see" (and it's pretty excessive in this doc).

[lamek]

Thanks @sfshaza2. I cleaned up the other instances in this doc.