Fix typos (and a few errors) in the API docs.

[brandondiamond]

Description

Thought I'd correct a few simple errors / typos in the API docs. This PR includes one or two larger changes and a whole mess of minor grammatical tweaks and fixes.

Related Issues

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Tests

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Checklist

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• I updated/added relevant documentation (doc comments with ///).
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[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. While there are exceptions to this rule, if this patch modifies code it is probably not an exception.

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

[gspencergoog]