Add docs for NOT VISIBLE syntax in schema specifications

[rmloveland]

In v21.1+ it's possible to specify in the schema that a column is NOT VISIBLE, which means it won't show up when you run SELECT * FROM t. This is an option whether or not the column is computed or has a different name. (This new NOT VISIBLE syntax can also be used in regular tables or multi-region tables.)

See also:

• Update "Computed Columns" page with examples of new multi-region columns and syntax #10401
• Document how to avoid the cost of optimizer uniqueness checks in multi-region settings #10402
• Document the non-computed ways to give the partitioned region column a different name #10403

[ericharmeling]

Hey Rich, I'm going to try to knock this out in my next "sprint". I'm tracking it here: #10646

I'm going to close this for now