[docs] Update common_issues.rst: update information about reveal type & locals

docs/source/common_issues.rst

@@ -504,12 +504,19 @@ to see the types of all local variables at once. Example:
    #     a: builtins.int
    #     b: builtins.str
 .. note::
-
-   ``reveal_type`` and ``reveal_locals`` are only understood by mypy and
-   don't exist in Python. If you try to run your program, you'll have to
-   remove any ``reveal_type`` and ``reveal_locals`` calls before you can
-   run your code. Both are always available and you don't need to import
-   them.
+   ``reveal_type`` and ``reveal_locals`` are understood by mypy during
+   typechecking, and don't have to be imported as functions. However,
+   if you don't import them, then they don't exist at runtime! Therefore,
+   you'll have to remove any ``reveal_type`` and ``reveal_locals`` calls
+   from your program or else Python will give you an error at runtime about
+   those names being undefined. Alternatively, you can import ``reveal_type``
+   from ``typing_extensions`` (or, in more recent versions of python, from
+   ``typing``) so its name will be defined at runtime. There is no analogous
+   fix for ``reveal_locals``. It simply must be removed from the code before
+   the code is run. (Although, technically, if you really didn't want to
+   remove those calls, you could use ``if not typing.TYPE_CHECKING:
+   reveal_locals = lambda: None`` or similar to define the function to
+   something else at runtime.)
 
 .. _silencing-linters: