Document the fact that that using a type ignore of a broader ignore when a subcode exists triggers unused ignore

docs/source/error_code_list2.rst

@@ -506,6 +506,10 @@ Example:
         # Python 3.8, or Python 3.9
         42 + "testing..."  # type: ignore
 
+If a narrower code (a subcode) is ignored by specifying a wider code, this counts
+as an instance of unused-ignore. (The wider code is, conceptually, partially unused.)
+For more information about subcodes, see :ref:`subcodes-of-error-codes`.
+
 .. _code-explicit-override:
 
 Check that ``@override`` is used when overriding a base class method [explicit-override]

docs/source/error_codes.rst

@@ -99,6 +99,8 @@ So one can e.g. enable some code globally, disable it for all tests in
 the corresponding config section, and then re-enable it with an inline
 comment in some specific test.
 
+.. _subcodes-of-error-codes:
+
 Subcodes of error codes
 -----------------------
 
@@ -110,6 +112,9 @@ is a subcode of another one, it will be mentioned in the documentation for the n
 code. This hierarchy is not nested: there cannot be subcodes of other
 subcodes.
 
+If a narrower code is ignored by specifying a wider code, this counts as an instance of
+:ref:`unused-ignore<code-unused-ignore>`. (The wider code is, conceptually, partially
+unused.)
 
 Requiring error codes
 ---------------------