Mention flat indentation 'match: expression'

pep-0622.rst

@@ -1168,7 +1168,19 @@ In sample programs using `match`, written as part of the development of this
 PEP, a noticeable improvement in code brevity is observed, more than making up
 for the additional indentation level.
 
-TODO: flat indentation with "match: expression" at the top.
+Another proposal considered was to use flat indentation but put the
+expression on the line after ``match:``, like this::
+
+  match:
+      expression
+  case pattern_1:
+      ...
+  case pattern_2:
+      ...
+
+This was ultimately rejected because the first block would be a
+novelty in Python's grammar: a block whose only content is a single
+expression rather than a sequence of statements.
 
 
 Alternatives for constant value pattern