Update extending_mypy.rst

[sobolevn]

Two things I have changed:

1. get_function_signature_hook was missing from docs, but it is defined

   mypy/mypy/plugin.py

   Lines 368 to 376 in ac66403

   	# A context for a function signature hook that infers a better signature for a
   	# function. Note that argument types aren't available yet. If you need them,
   	# you have to use a method hook instead.
   	FunctionSigContext = NamedTuple(
   	'FunctionSigContext', [
   	('args', List[List[Expression]]), # Actual expressions for each formal argument
   	('default_signature', CallableType), # Original signature of the method
   	('context', Context), # Relevant location context (e.g. for error messages)
   	('api', CheckerPluginInterface)])

2. While reading the docs, this UserDefined for get_class_decorator_hook example caught my eye. It was not clear how UserDefined and @customize work. I believe that @dataclass example is better for two reasons. First, it is an existing API and is already familiar to our users. Secondly, there's an actual plugin for it, so people can see how it is implemented

Refs #6760

[JelleZijlstra]

Thank you!