bpo-37602: Clarify that the lib2to3 nonzero fixer changes only definitions (GH-30075) (GH-30083)

(cherry picked from commit 481f3ff)

Co-authored-by: Irit Katriel <[email protected]>

Author: miss-islington

Doc/library/2to3.rst

@@ -333,7 +333,8 @@ and off individually.  They are described here in more detail.
 
 .. 2to3fixer:: nonzero
 
-   Renames :meth:`__nonzero__` to :meth:`~object.__bool__`.
+   Renames definitions of methods called :meth:`__nonzero__`
+   to :meth:`~object.__bool__`.
 
 .. 2to3fixer:: numliterals