Clarify "Using types...but not at runtime" docs (#15029)

The section "Using classes that are generic in stubs but not at runtime"
on the runtime_troubles.html page is very helpful, but naive readers who
follow its instructions will almost inevitably create a runtime
`NameError`. This PR updates the example to include an `annotations`
import that will avert such a `NameError`.

Author: jonshea

docs/source/runtime_troubles.rst

@@ -277,10 +277,18 @@ sections, these can be dealt with by using :ref:`typing.TYPE_CHECKING
 
 .. code-block:: python
 
+   from __future__ import annotations
    from typing import TYPE_CHECKING
    if TYPE_CHECKING:
        from _typeshed import SupportsRichComparison
 
+    def f(x: SupportsRichComparison) -> None
+
+The ``from __future__ import annotations`` is required to avoid
+a ``NameError`` when using the imported symbol.
+For more information and caveats, see the section on
+:ref:`future annotations <future-annotations>`.
+
 .. _generic-builtins:
 
 Using generic builtins