Revise assert_never

This revises the docstring of git.types.assert_never to more
clearly express its semantics and usage, to use the type names used
in the typing module, typing_extensions package, and mypy. This
expands some parts when doing so seemed to benefit clarity.

The docstring had previously said AssertionError was raised, but
the code raised ValueError. For now I've adjusted the docstring to
be accurate to the code's behavior, but maybe this should change.

I also removed the part about the mypy error being on variable
creation, since I am not clear on what that was referring to, and
if it means the first name binding operation for a variable in its
scope, then I am not sure why that would be where an error message
would be expected when using assert_never.

Finally, this slightly adjusts the message:

- "Literal" is decapitalized, to decrease confusion if the default
  message is used when matching on something not a typing.Literal.

- The exception message prints the unexpected value's repr rather
  than its str, since the repr, when different from the str, is
  usually the representation more useful for debugging.

Author: EliahKagan

git/types.py

@@ -72,19 +72,27 @@
 
 
 def assert_never(inp: NoReturn, raise_error: bool = True, exc: Union[Exception, None] = None) -> None:
-    """For use in exhaustive checking of literal or Enum in if/else chain.
+    """For use in exhaustive checking of a literal or enum in if/else chains.
 
-    Should only be reached if all members not handled OR attempt to pass non-members through chain.
+    A call to this function should only be reached if not all members are handled, or if
+    an attempt is made to pass non-members through the chain.
 
-    If all members handled, type is Empty. Otherwise, will cause mypy error.
+    :param inp:
+        If all members are handled, the argument for `inp` will have the
+        :class:`~typing.Never`/:class:`~typing.NoReturn` type. Otherwise, the type will
+        mismatch and cause a mypy error.
 
-    If non-members given, should cause mypy error at variable creation.
+    :param raise_error:
+        If ``True``, will also raise :class:`ValueError` with a general "unhandled
+        literal" message, or the exception object passed as `exc`.
 
-    If raise_error is True, will also raise AssertionError or the Exception passed to exc.
+    :param exc:
+        It not ``None``, this should be an already-constructed exception object, to be
+        raised if `raise_error` is ``True``.
     """
     if raise_error:
         if exc is None:
-            raise ValueError(f"An unhandled Literal ({inp}) in an if/else chain was found")
+            raise ValueError(f"An unhandled literal ({inp!r}) in an if/else chain was found")
         else:
             raise exc