diff --git a/docs/source/command_line.rst b/docs/source/command_line.rst
index c1b757a00ef2..edcb420f03c9 100644
--- a/docs/source/command_line.rst
+++ b/docs/source/command_line.rst
@@ -101,7 +101,7 @@ Optional arguments
 
     Show program's version number and exit.
 
-.. option:: -O FORMAT, --output FORMAT {json}
+.. option:: -O {json}, --output {json}
 
     Set a custom output format.
 
diff --git a/mypy/main.py b/mypy/main.py
index b543cd33fe44..816e5b858b9d 100644
--- a/mypy/main.py
+++ b/mypy/main.py
@@ -568,11 +568,7 @@ def add_invertible_flag(
     )
 
     general_group.add_argument(
-        "-O",
-        "--output",
-        metavar="FORMAT",
-        help="Set a custom output format",
-        choices=OUTPUT_CHOICES,
+        "-O", "--output", help="Set a custom output format", choices=sorted(OUTPUT_CHOICES)
     )
 
     config_group = parser.add_argument_group(
@@ -1108,6 +1104,8 @@ def add_invertible_flag(
     parser.add_argument(
         "--enable-incomplete-feature",
         action="append",
+        # We can't just use choices= here because we actually accept more than this;
+        # see process_incomplete_features.
         metavar="{" + ",".join(sorted(INCOMPLETE_FEATURES)) + "}",
         help="Enable support of incomplete/experimental features for early preview",
     )