Add field explanations for TiDB mysql schema tables

[qiancai]

Issue

The TiDB's mysql Schema document lists TiDB system tables but lacks detailed explanations of the fields within these tables. These explanations are crucial for users to comprehend these tables, particularly tables that are not compatible with MySQL, such as stats_histograms.

Request

Add field explanations for mysql schema tables.

[dveeden]

When we do this we need to make clear that the best practice is to use statement instead of directly inspecting system tables where possible. So SHOW CREATE USER ... instead of SELECT ... FROM mysql.user... etc. This is true for both MySQL, TiDB and other databases.