fast analyze does not have an experimental warning

[morgo]

Error Report

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

1. What is the URL/path of the document related to this issue?

See: https://docs.pingcap.com/tidb/stable/system-variables#tidb_enable_fast_analyze

2. How would you like to improve it?

The experimental warning is mentioned here:
https://docs.pingcap.com/tidb/stable/sql-statement-analyze-table#mysql-compatibility

But it should be mentioned for the sysvar as well, since most other experimental sysvars mention it on the sysvars page.