Clarify difference between disallow_untyped_defs and disallow_incompl…

…ete_defs (#15247)

Fixes #9963, fixes #12693

docs/source/command_line.rst

@@ -353,12 +353,17 @@ definitions or calls.
 .. option:: --disallow-untyped-defs
 
     This flag reports an error whenever it encounters a function definition
-    without type annotations.
+    without type annotations or with incomplete type annotations.
+    (a superset of :option:`--disallow-incomplete-defs`).
+
+    For example, it would report an error for :code:`def f(a, b)` and :code:`def f(a: int, b)`.
 
 .. option:: --disallow-incomplete-defs
 
     This flag reports an error whenever it encounters a partly annotated
-    function definition.
+    function definition, while still allowing entirely unannotated definitions.
+
+    For example, it would report an error for :code:`def f(a: int, b)` but not :code:`def f(a, b)`.
 
 .. option:: --check-untyped-defs
 

docs/source/config_file.rst

@@ -498,14 +498,19 @@ section of the command line docs.
     :default: False
 
     Disallows defining functions without type annotations or with incomplete type
-    annotations.
+    annotations (a superset of :confval:`disallow_incomplete_defs`).
+
+    For example, it would report an error for :code:`def f(a, b)` and :code:`def f(a: int, b)`.
 
 .. confval:: disallow_incomplete_defs
 
     :type: boolean
     :default: False
 
-    Disallows defining functions with incomplete type annotations.
+    Disallows defining functions with incomplete type annotations, while still
+    allowing entirely unannotated definitions.
+
+    For example, it would report an error for :code:`def f(a: int, b)` but not :code:`def f(a, b)`.
 
 .. confval:: check_untyped_defs
 

mypy/main.py

@@ -696,7 +696,8 @@ def add_invertible_flag(
         "--disallow-incomplete-defs",
         default=False,
         strict_flag=True,
-        help="Disallow defining functions with incomplete type annotations",
+        help="Disallow defining functions with incomplete type annotations "
+        "(while still allowing entirely unannotated definitions)",
         group=untyped_group,
     )
     add_invertible_flag(