pythongh-119580: Improve version added section for convenience variable (python#119583)

Author: gaogaotiantian

Doc/library/pdb.rst

@@ -290,6 +290,8 @@ There are three preset *convenience variables*:
 
 .. versionadded:: 3.12
 
+ Added the *convenience variable* feature.
+
 .. index::
  pair: .pdbrc; file
  triple: debugger; configuration; file