[docs] add more info to allow_redefinition

Author: KotlinIsland

docs/source/command_line.rst

@@ -516,6 +516,9 @@ of the above sections.
     unrelated type. This flag enables redefinition of a variable with an
     arbitrary type *in some contexts*: only redefinitions within the
     same block and nesting depth as the original definition are allowed.
+
+    The expression for the new value must contain a reference to the variable.
+
     Example where this can be useful:
 
     .. code-block:: python
@@ -524,7 +527,8 @@ of the above sections.
            # 'items' has type list[str]
            items = [item.split() for item in items]
            # 'items' now has type list[list[str]]
-           ...
+
+           items = "mypy"  # invalid redefinition to str because the expression doesn't contain a reference to 'items'
 
 .. option:: --local-partial-types
 

docs/source/config_file.rst

@@ -614,6 +614,17 @@ section of the command line docs.
     Allows variables to be redefined with an arbitrary type, as long as the redefinition
     is in the same block and nesting level as the original definition.
 
+    The expression for the new value must contain a reference to the variable:
+
+    .. code-block:: python
+
+       def process(items: list[str]) -> None:
+           # 'items' has type list[str]
+           items = [item.split() for item in items]
+           # 'items' now has type list[list[str]]
+
+           items = "mypy"  # invalid redefinition to str because the expression doesn't contain a reference to 'items'
+
 .. confval:: local_partial_types
 
     :type: boolean