system-variables: add documents for error_count and max_connections (#10525)

[ti-chi-bot]

This is an automated cherry-pick of #10525

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What is changed, added or deleted? (Required)

As stated in the title, add missing documents for 2 sys vars. I also would like to add some more descriptions for ssl_cert, ssl_key and ssl_ca.

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 24214f0