address review: start new paragraph for invariance error example

Made-with: Cursor

Author: Leo Ji

docs/source/common_issues.rst

@@ -307,8 +307,10 @@ Most mutable generic collections are invariant. When using the legacy
 ``TypeVar`` syntax, mypy considers all user-defined generic classes invariant
 by default (see :ref:`variance-of-generics` for motivation). When using the
 :pep:`695` syntax (``class MyClass[T]: ...``), variance is inferred from
-usage rather than defaulting to invariant. This could lead to some unexpected
-errors when combined with type inference. For example:
+usage rather than defaulting to invariant.
+
+The fact that mutable sequences are usually invariant can lead to some
+unexpected errors when combined with type inference. For example:
 
 .. code-block:: python