Improve documentation on verified_* testing methods

[alamb]

As I have been reviewing PRs and working on #827 I have always wanted slightly more specific documentation on the testing methods

[coveralls]

Pull Request Test Coverage Report for Build 4345790591

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.187%

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Totals
Change from base Build 4344757158:	0.0%
Covered Lines:	13565
Relevant Lines:	15739

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💛 - Coveralls

[ankrgyl]

Wow, thank you for writing this up!

A wishlist item for me would be a quick guide (even just bullet points) that defines what tests you should add for your change (e.g. add a sanity test, a roundtrip test, etc.).

[alamb]

A wishlist item for me would be a quick guide (even just bullet points) that defines what tests you should add for your change (e.g. add a sanity test, a roundtrip test, etc.).

Yes, that would be great. If you have time that would be great. Maybe we could add it to the contributing section of the README https://github.com/sqlparser-rs/sqlparser-rs/blob/main/README.md#contributing