`callable` is not documented in type narrowing section

[sobolevn]

https://mypy.readthedocs.io/en/latest/type_narrowing.html

Example:

from typing import Callable, Union

x: Union[int, Callable[[], int]]

if callable(x):
    reveal_type(x)  # N: Revealed type is "def () -> builtins.int"
else:
    reveal_type(x)  # N: Revealed type is "builtins.int"

Related #11088
Related #11150