1. not list() -> callable()
The args of hooking parameters of Sanic have to be callables.
For wrong parameters, errors will be generated from:
```
listeners += args
```
By checking just list type, the raised error will be associated
with `[args]` instead of `args`, which is not given by users.
With this patch, the raised error will be associated with `args`.
Then users can notice their argument was neither callable nor list
in the easier way.
2. Function -> Functions in document
Regarding the parameter as a list is harmless to the user code.
But unawareness of its type can be list can limit the potent of
the user code.
15e7d8ab2e