Better message for "Function ... could always be true"

Consider this example:

def f(x: int) -> str: pass

if f:
    pass

It generates this error:

t.py:3: error: Function "Callable[[int], str]" could always be true in boolean context  [truthy-function]

Instead, if we have a direct reference to a function definition, we could generate a message like this which would be clearer, especially if the signature is complex:

t.py:3: error: Function "f" could always be true in boolean context  [truthy-function]

There are at least these different cases to consider:

1. Direct reference to a function using a short name of a function (similar to the above example).
2. Direct reference to a function using a module prefix (e.g. if mod.func:).
3. Reference to a method (e.g. if obj.method:).
4. Reference to a variable/attribute (Var node) with type Callable[...] (in this case we could show the name of the variable/attribute or the callable type -- both would be okay).
5. Any other expression that produces a callable value, such as if func(): where func returns a callable. Here the best option is to show the callable type as we do now.

Empieza reproduciendo el ejemplo e inspecciona cómo el diagnóstico [truthy-function] formatea las expresiones invocables. Compara referencias directas a funciones, funciones calificadas por módulo, métodos, variables o atributos invocables y expresiones que devuelven objetos invocables. La tarea estará terminada cuando las referencias directas usen el nombre de función más claro cuando corresponda, mientras que los demás casos conserven un tipo o una descripción útil del objeto o la expresión invocable.