docs/array: Document array module.

docs/library/array.rst

+:mod:`array` -- arrays of numeric data
+======================================
+
+.. module:: array
+   :synopsis: efficient arrays of numeric data
+
+See `Python array <https://docs.python.org/3/library/array.html>`_ for more
+information.
+
+Supported format codes: ``b``, ``B``, ``h``, ``H``, ``i``, ``I``, ``l``,
+``L``, ``q``, ``Q``, ``f``, ``d`` (the latter 2 depending on the
+floating-point support).
+
+Classes
+-------
+
+.. class:: array.array(typecode, [iterable])
+
+    Create array with elements of given type. Initial contents of the
+    array are given by an `iterable`. If it is not provided, an empty
+    array is created.
+
+    .. method:: append(val)
+
+        Append new element to the end of array, growing it.
+
+    .. method:: extend(iterable)
+
+        Append new elements as contained in an iterable to the end of
+        array, growing it.