Document the lifetime of `PyUnicode_AsUTF8String` (#92325)

The current wording implied this, but didn't state it explicitly.

Doc/c-api/unicode.rst

@@ -1075,7 +1075,8 @@ These are the UTF-8 codec APIs:
 
    This caches the UTF-8 representation of the string in the Unicode object, and
    subsequent calls will return a pointer to the same buffer.  The caller is not
-   responsible for deallocating the buffer.
+   responsible for deallocating the buffer. The buffer is deallocated and
+   pointers to it become invalid when the Unicode object is garbage collected.
 
    .. versionadded:: 3.3