[Docs] Clarify that Flex does not wrap

[TzviPM]

The Flex widget, despite its name, differs from the behaviour of flexbox in a web setting. In particular, people coming from a web background may be used to using flex-wrap: wrap from CSS, which Flex does not support. This PR attempts to elide this confusion by pointing to the Wrap widget from the documentation for Flex.

Although these additions are aiming to avoid confusion with flexbox, I was careful not to specifically mention the web use case, so as not to further confuse anyone who wouldn't have otherwise been confused.

Fixes #113403 113403

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[TzviPM]

My editor seems to have auto formatted the code here. If desired, I will amend this tomorrow using "save without formatting"

[TzviPM]

Also, re:tests, these changes are docs-only.

[goderbauer]

LGTM