Add the note as a CPython implementation detail

Author: vstinner

Doc/c-api/bytes.rst

@@ -8,9 +8,11 @@ Bytes Objects
 These functions raise :exc:`TypeError` when expecting a bytes parameter and
 called with a non-bytes parameter.
 
-A :c:type:`PyBytesObject` allocates an extra trailing null byte for
-compatibility with null terminated C strings. It is not counted in
-:c:func:`PyBytes_Size` length.
+.. impl-detail::
+
+   A :c:type:`PyBytesObject` allocates an extra trailing null byte for
+   compatibility with null terminated C strings. It is not counted in
+   :c:func:`PyBytes_Size` length.
 
 .. index:: pair: object; bytes