gh-156939: Document that PyBytesObject ends with a NUL byte

Author: vstinner

Doc/c-api/bytes.rst

@@ -8,6 +8,10 @@ Bytes Objects
 These functions raise :exc:`TypeError` when expecting a bytes parameter and
 called with a non-bytes parameter.
 
+.. impl-detail::
+   A :c:type:`PyBytesObject` allocates an extra trailing NUL byte for
+   compatibility with NUL terminated C strings.
+
 .. index:: pair: object; bytes