Add spatial data types to MyRocks limitations

[The MyRocks limitations page|https://www.percona.com/doc/percona-server/5.7/myrocks/limitations.html] mentions only spatial indexes as currently unsupported.

In fact, all spatial data types described at https://dev.mysql.com/doc/refman/5.7/en/spatial-type-overview.html are not currently supported by MyRocks. I think this limitation is important enough to document explicitly.

I also don't think this limitation is even covered in the test suite.