system-variables: add documents for error_count and max_connections

[bb7133]

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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
• dveeden

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[TomShawn]

rest LGTM

[TomShawn]

LGTM

[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: 27988cf

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10651.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10653.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10654.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10655.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10656.