Document new import error codes (#15840)

See #14740

My PR was pretty old and predates the nice check to ensure error codes
are documented.

docs/source/error_code_list.rst

@@ -648,8 +648,18 @@ the issue:
 
 .. _code-import:
 
-Check that import target can be found [import]
-----------------------------------------------
+Check for an issue with imports [import]
+----------------------------------------
+
+Mypy generates an error if it can't resolve an `import` statement.
+This is a parent error code of `import-not-found` and `import-untyped`
+
+See :ref:`ignore-missing-imports` for how to work around these errors.
+
+.. _code-import-not-found:
+
+Check that import target can be found [import-not-found]
+--------------------------------------------------------
 
 Mypy generates an error if it can't find the source code or a stub file
 for an imported module.
@@ -658,11 +668,31 @@ Example:
 
 .. code-block:: python
 
-    # Error: Cannot find implementation or library stub for module named 'acme'  [import]
-    import acme
+    # Error: Cannot find implementation or library stub for module named "m0dule_with_typo"  [import-not-found]
+    import m0dule_with_typo
 
 See :ref:`ignore-missing-imports` for how to work around these errors.
 
+.. _code-import-untyped:
+
+Check that import target can be found [import-untyped]
+--------------------------------------------------------
+
+Mypy generates an error if it can find the source code for an imported module,
+but that module does not provide type annotations (via :ref:`PEP 561 <installed-packages>`).
+
+Example:
+
+.. code-block:: python
+
+    # Error: Library stubs not installed for "bs4"  [import-untyped]
+    import bs4
+    # Error: Skipping analyzing "no_py_typed": module is installed, but missing library stubs or py.typed marker  [import-untyped]
+    import no_py_typed
+
+In some cases, these errors can be fixed by installing an appropriate
+stub package. See :ref:`ignore-missing-imports` for more details.
+
 .. _code-no-redef:
 
 Check that each name is defined once [no-redef]