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

docs/source/common_issues.rst

@@ -505,11 +505,18 @@ to see the types of all local variables at once. Example:
    #     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``
+    (or, in versions of python older than 3.11, from ``typing_extensions``) 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: