docs: correct how to set null treatment on a window and list array_first

The Null Treatment section said null treatment needs the builder, but
its own example passes it in the Window, and over() now keeps one set on
the aggregate. The higher-order functions list in expressions.md also
missed array_first.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: timsaucer

docs/source/user-guide/common-operations/expressions.md

@@ -157,7 +157,9 @@ In this example, the `repeated_array` column will contain `[[1, 2, 3], [1, 2, 3]
 Some array functions take a *lambda function*: a small function that runs once
 per element. {py:func}`~datafusion.functions.array_transform` maps a lambda over
 every element, {py:func}`~datafusion.functions.array_filter` keeps the elements
-for which a predicate lambda is true, and
+for which a predicate lambda is true,
+{py:func}`~datafusion.functions.array_first` returns the first element that
+satisfies a predicate lambda, and
 {py:func}`~datafusion.functions.array_any_match` returns whether any element
 satisfies a predicate lambda. (Functions that take another function as an
 argument are sometimes called *higher-order* functions.)
@@ -173,6 +175,7 @@ ctx = SessionContext()
 df = ctx.from_pydict({"a": [[1, 2, 3], [4, 5]]})
 df.select(f.array_transform(col("a"), lambda v: v * 2).alias("doubled"))
 df.select(f.array_filter(col("a"), lambda v: v > 2).alias("big_only"))
+df.select(f.array_first(col("a"), lambda v: v > 2).alias("first_big"))
 df.select(f.array_any_match(col("a"), lambda v: v > 3).alias("has_big"))
 ```
 

docs/source/user-guide/common-operations/windows.md

@@ -181,8 +181,8 @@ f.sum(col("v")).over(Window(order_by=col("v"), window_frame=whole))
 ### Null Treatment
 
 When using aggregate functions as window functions, it is often useful to specify how null values
-should be treated. In order to do this you need to use the builder function. In future releases
-we expect this to be simplified in the interface.
+should be treated. Pass `null_treatment` in the `Window`, or set it on the aggregate itself, which
+`over()` keeps (see {ref}`aggregate_over_options`).
 
 One common usage for handling nulls is the case where you want to find the last value up to the
 current row. In the following example we demonstrate how setting the null treatment to ignore