docs: publish the Function pages only once Functions launch

The two Function pages declare `launch: functions`, and every other page
marks what it says about Functions, so the assembled site leaves them out
until the launch. Before it, the pages that offered Functions for Python
work point Enterprise readers at Geneva instead.

The SQL Function example now uses Arrow type names, which the server
requires, and states the 3.12 default for `python_version`.

docs/web/sql/ddl/table.mdx

@@ -30,8 +30,8 @@ The table's columns and types, without a plan -- the SQL counterpart of
 
 ## ALTER TABLE {#alter}
 
-`ALTER TABLE ... ADD COLUMN` covers three different things, distinguished by
-what follows the column name.
+What `ALTER TABLE ... ADD COLUMN` does depends on what follows the column
+name.
 
 ### A plain column {#plain-column}
 
@@ -52,6 +52,7 @@ The column is committed empty and the expression is stored, not evaluated --
 declaring one on a billion-row table costs what declaring one on an empty table
 costs. [`REFRESH COLUMN`](/sql/dml/refresh) fills it.
 
+{/* if functions launched */}
 ### A function column {#function-column}
 
 ```sql
@@ -86,6 +87,7 @@ ALTER TABLE t ADD COLUMN c AS f(x) WITH (num_workers = '2')
 `WITH` on a plain computed column accepts only `version`. Resource options such
 as `num_workers` apply to function columns, where there are workers to size.
 </Note>
+{/* end if */}
 
 ### Table properties {#properties}