[libc++] Reorganize the documentation of extensions for integral types

Author: ldionne

libcxx/docs/UsingLibcxx.rst

@@ -424,32 +424,30 @@ which no dialect declares as such (See the second form described above).
 * ``to_integer``
 * ``to_underlying``
 
-Additional types supported in random distributions
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-The `C++ Standard <http://eel.is/c++draft/rand#req.genl-1.5>`_ mentions that instantiating several random number
-distributions with types other than ``short``, ``int``, ``long``, ``long long``, and their unsigned versions is
-undefined. As an extension, libc++ supports instantiating ``binomial_distribution``, ``discrete_distribution``,
-``geometric_distribution``, ``negative_binomial_distribution``, ``poisson_distribution``, and ``uniform_int_distribution``
-with ``int8_t``, ``__int128_t`` and their unsigned versions.
-
 Extended integral type support
 ------------------------------
 
-Several platforms support the 128-bit integral types ``__int128_t`` and
-``__uint128_t``. When these types are present they can be used in the headers
-as required by the Standard:
+Several platforms support types that are not specified in the Standard, such as
+the 128-bit integral types ``__int128_t`` and ``__uint128_t``. As an extension,
+libc++ does a best-effort attempt to support these types like other integral
+types, by supporting them notably in:
 
 * ``<bits>``
 * ``<charconv>``
 * ``<functional>``
 * ``<type_traits>``
-
-As an extension these types can be used in the following headers:
-
 * ``<format>``
 * ``<random>``
 
+Additional types supported in random distributions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The `C++ Standard <http://eel.is/c++draft/rand#req.genl-1.5>`_ mentions that instantiating several random number
+distributions with types other than ``short``, ``int``, ``long``, ``long long``, and their unsigned versions is
+undefined. As an extension, libc++ supports instantiating ``binomial_distribution``, ``discrete_distribution``,
+``geometric_distribution``, ``negative_binomial_distribution``, ``poisson_distribution``, and ``uniform_int_distribution``
+with ``int8_t``, ``__int128_t`` and their unsigned versions.
+
 Extensions to ``<format>``
 --------------------------