dm: add description about mysql-instance.meta

[GMHDBJD]

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[lance6716]

需要强调一下，meta 项整个不存在，而不是 meta 项是空的、下面的 field 不存在

[shichun-0415]

LGTM

[shichun-0415]

/merge

[ti-chi-bot]

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

Commit hash: c91f5fd

[shichun-0415]

/hold

[shichun-0415]

/merge