[3.15] gh-156939: Document that PyBytesObject ends with a NUL byte (GH-157236) (#157550)

Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Petr Viktorin <encukou@gmail.com>

Author: 3 people

Doc/c-api/bytearray.rst

@@ -12,6 +12,12 @@ Byte Array Objects
 
    This subtype of :c:type:`PyObject` represents a Python bytearray object.
 
+   .. impl-detail::
+
+      The internal buffer of :c:type:`PyByteArrayObject` always includes an
+      extra trailing null byte for compatibility with null terminated C
+      strings.  This extra byte is not counted in :c:func:`PyByteArray_Size`
+      nor in the *len* arguments of the functions below.
 
 .. c:var:: PyTypeObject PyByteArray_Type
 

Doc/c-api/bytes.rst

@@ -8,6 +8,13 @@ Bytes Objects
 These functions raise :exc:`TypeError` when expecting a bytes parameter and
 called with a non-bytes parameter.
 
+.. impl-detail::
+
+   The internal buffer of :c:type:`PyBytesObject` always includes an extra
+   trailing null byte for compatibility with null terminated C strings.
+   This extra byte is not counted in :c:func:`PyBytes_Size` nor in the
+   various *length* and *size* arguments of the functions below.
+
 .. index:: pair: object; bytes