Improve docstrings on Module class, workflow strategies, and grouper function

[FernandoCelmer]

Description

When a user passes the wrong type to Config, the error message is misleading.

The problem

Config(storage="not a storage")
# NotCallableObject: Problem validating the 'storage' object type; this is not a callable object

The message says "not a callable object" but the real problem is "wrong type". The user thinks: "but I didn't try to call anything?"

The fix

A clearer message:

# Before
"Problem validating the 'storage' object type; this is not a callable object"

# After
"'storage' must be an instance of Storage, got str instead"

How to implement

1. In dotflow/core/exception.py — update NotCallableObject message or create a new InvalidProviderType exception:

   class InvalidProviderType(DotFlowException):
       def __init__(self, name: str, expected: type, got: type):
           super().__init__(f"'{name}' must be an instance of {expected.__name__}, got {got.__name__} instead")

2. In dotflow/core/config.py — update the _validate() call:

   if value is not None and not isinstance(value, abc):
       raise InvalidProviderType(name=name, expected=abc, got=type(value))

3. Small change in 2 files, improves error UX for all users