attempt to fix incorrect examples in 'How to Teach This'

Author: adqm

peps/pep-0798.rst

@@ -559,7 +559,8 @@ by a similar analogy::
     for x in it:
         out[k_expr] = v_expr
 
-    # equivalent to out = {**expr for x in it}
+    # equivalent to out = {**expr for x in it}, provided that expr evaluates to
+    # a mapping that can be unpacked with **
     out = {}
     for x in it:
         out.update(expr)
@@ -597,7 +598,7 @@ Similarly, we can think of ``out = {...x... for x in it}`` as equivalent to the
 following code, regardless of whether or not ``...x...`` uses ``*`` or ``**``
 or ``:``::
 
-    out = set()
+    out = set()  # or out = {}
     for x in it:
         out.update({...x...})