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/function.mdx

@@ -4,6 +4,7 @@ sidebarTitle: "Function"
 description: "CREATE FUNCTION, SHOW FUNCTIONS, and DROP FUNCTION."
 icon: function
 keywords: ["create function", "drop function", "show functions", "sql", "udf"]
+launch: functions
 ---
 
 <Badge color="red">Enterprise</Badge> [SQL runs on a deployment.](/sql)
@@ -11,12 +12,17 @@ keywords: ["create function", "drop function", "show functions", "sql", "udf"]
 ## CREATE FUNCTION {#create}
 
 ```sql
-CREATE FUNCTION f(x BIGINT) RETURNS BIGINT
+CREATE FUNCTION f(x int64) RETURNS int64
 LANGUAGE python
 AS 'def f(x): return x * 2'
 WITH (pip = 'numpy==1.26.4', python_version = '3.11')
 ```
 
+Parameter and result types are Arrow type names, not SQL ones: `int64`,
+`float64`, `utf8`, `bool`, `date32`, and lists such as `list<float32>` or
+`fixed_size_list<float32, 768>`. `BIGINT` is refused. A parameter accepts nulls
+unless it is declared `NOT NULL`.
+
 The body is Python source. Its dependencies are declared with it and are part
 of the **immutable function version** -- changing one produces a new version
 rather than altering the registered one, so a column filled by an earlier
@@ -27,7 +33,7 @@ version is unaffected.
 | `pip` | pip requirements for the worker environment |
 | `conda` | conda packages instead of pip |
 | `conda_channels` | channels in priority order; requires `conda` |
-| `python_version` | worker Python version, defaulting to yours |
+| `python_version` | worker Python version; `3.12` when omitted, whatever Python you send the statement from |
 | `gpu` | `'true'` when every execution needs a GPU |
 
 **Errors and limits**