Document 'admin show slow' command

[morgo]

See: pingcap/tidb#7785

Should be documented here: https://pingcap.com/docs/sql/slow-query/

[lilin90]

@tiancaiamao Would you please help update the "Slow Query Log" document? Thanks!
https://pingcap.com/docs/sql/slow-query/ OR https://pingcap.com/docs-cn/sql/slow-query/