Add extends_documentation_fragment support in AnsibleArgSpecValidator

[rayrayibm]

Hello,

Today AnsibleArgSpecValidator ignores "extends_documentation_fragment," which means you cannot use doc fragments for validating and thus need to add the fragment in multiple modules/plugins.

I know this is something that is likely planned since it is mentioned as a TODO in the argspec_validate.py code. Is there a potential timeline? I would consider working on it myself, but I'm a little unfamiliar with the code to know how best to add this. If someone can point me in the right direction, I can give it a go.

Thanks
Ray