docs: clarify that invariant-by-default applies to legacy TypeVar syntax (#21108)

Closes #20366.

The "invariant by default" rule applies to the legacy `TypeVar` syntax.
Under PEP 695 (`class MyClass[T]: ...`), mypy infers variance from usage
instead. Updated the wording in `common_issues.rst` to make this
distinction clear.

---------

Co-authored-by: Leo Ji <nuglifeleoji@gmail.com>

Author: nuglifeleoji

docs/source/common_issues.rst

@@ -303,9 +303,13 @@ See :ref:`type-narrowing` for more information.
 Invariance vs covariance
 ------------------------
 
-Most mutable generic collections are invariant, and mypy considers all
-user-defined generic classes invariant by default
-(see :ref:`variance-of-generics` for motivation). This could lead to some
+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.
+
+The fact that mutable sequences are usually invariant can lead to some
 unexpected errors when combined with type inference. For example:
 
 .. code-block:: python