Document follow_untyped_imports config option & flag

Author: DeinAlptraum

docs/source/command_line.rst

@@ -166,6 +166,10 @@ imports.
 
     For more details, see :ref:`ignore-missing-imports`.
 
+.. option:: --follow-untyped-imports
+
+    This flag makes mypy analyze imports without stubs or a py.typed marker.
+
 .. option:: --follow-imports {normal,silent,skip,error}
 
     This flag adjusts how mypy follows imported modules that were not

docs/source/config_file.rst

@@ -315,6 +315,18 @@ section of the command line docs.
     match the name of the *imported* module, not the module containing the
     import statement.
 
+.. confval:: follow_untyped_imports
+
+    :type: boolean
+    :default: False
+
+    Typechecks imports from modules that do not have stubs or a py.typed marker.
+
+    If this option is used in a per-module section, the module name should
+    match the name of the *imported* module, not the module containing the
+    import statement. Note that scanning all unannotated modules might
+    significantly increase the runtime of your mypy calls.
+
 .. confval:: follow_imports
 
     :type: string

docs/source/running_mypy.rst

@@ -321,6 +321,12 @@ not catch errors in its use.
     recommend avoiding ``--ignore-missing-imports`` if possible: it's equivalent
     to adding a ``# type: ignore`` to all unresolved imports in your codebase.
 
+4.  To make mypy typecheck imports from modules without stubs or a py.typed
+    marker, you can set the :option:`--follow-untyped-imports <mypy --follow-untyped-imports>`
+    command line flag or set the :confval:`follow_untyped_imports` config file option to True,
+    either in the global section of your mypy config file, or individually on a
+    per-module basis.
+
 
 Library stubs not installed
 ---------------------------