Add column descriptions for INFORMATION_SCHEMA.PARTITIONS

[mjonss]

Change Request

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs. https://docs.pingcap.com/tidb/stable/information-schema-partitions only shows the table and its columns, not what the columns mean.

2. Describe your suggestion or addition. For each column describe the content, I was recently asked for how to get the partition limits/values for RANGE partitioning :) Which is in PARTITION_DESCRIPTION Similar to how https://docs.pingcap.com/tidb/stable/information-schema-columns is documented

3. Provide some reference materials (such as documents and websites) if you could. https://dev.mysql.com/doc/refman/8.0/en/information-schema-partitions-table.html