Update RefreshIndicator documentation to clarify the need for the child to be scrollable

[perclasson]

Description

Update RefreshIndicator documentation to clarify the need for the child to be scrollable. Screenshot from documentation:

Related Issues

Closes #22180

Tests

N/A, just updating documentation

Checklist

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• All existing and new tests are passing.
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Breaking Change

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[fluttergithubbot]

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.

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

[rami-a]

LGTM

[HansMuller]

LGTM with some suggestions about the doc.