document standard_compliant_cte system variable

Documentation saying that it conforms to standards is a bit weak as I don't suspect users know how to find/search SQL standards.

This also came up with comments on https://mariadb.com/kb/en/recursive-common-table-expressions-overview/#comment_4520

It came up in discussion on https://stackoverflow.com/questions/79629831/one-json-object-as-a-result-of-sql-recursive-cte

code comments could form the basis of this
https://github.com/MariaDB/server/blob/10.5/sql/sql_cte.cc#L1397-L1409