handlers: typing and doc fix

[adekmaulana]

There's inconsistency between using callable and Callable so the typing will be giving some error incompatible.
Because on ~Handler used Callable while on every type of handlers using callable.

callable: obj -> bool
Callable[*args: Unknown, **kwargs: Unknown] -> Unknown

also fixed some doc.

Before

(class) MessageHandler(callback: (__obj: object) -> bool, filters: Unknown = None)
The base class of the class hierarchy.

When called, it accepts no arguments and returns a new featureless instance that has no instance attributes and cannot be given any.

Typing Error

Argument of type "(client: Client, msg: Message) -> Coroutine[Any, Any, None]" cannot be assigned to parameter "callback" of type "(__obj: object) -> bool" in function "__init__"
  Type "(client: Client, msg: Message) -> Coroutine[Any, Any, None]" cannot be assigned to type "(__obj: object) -> bool"
    Parameter 1: type "object" cannot be assigned to type "Client"
      "object" is incompatible with "Client"
    Function accepts too few positional parameters; expected 2 but received 1
    Keyword parameter "msg" is missing in destination
    Function return type "Coroutine[Any, Any, None]" is incompatible with type "bool"
      "Coroutine[Any, Any, None]" is incompatible with "bool"

After:

(class) MessageHandler(callback: (_p0: Client, _p1: Message) -> Any, filters: Filter | None = None)
The base class of the class hierarchy.

When called, it accepts no arguments and returns a new featureless instance that has no instance attributes and cannot be given any.

Typing Error:
None