add more explains and examples for SELECT ... INTO OUTFILE statements

[qw4990]

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/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.4: #15507.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #15508.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #15509.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #15510.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.3: #15511.