Docs: Rephrase v_args() documentation to underline it only applies to Transformer classes

[skepppy]

Related to issue #1457, rewording the documentation of v_args() to specify it is only applicable to Transformer classes.

[erezsh]

I think we should avoid using the word visitor at all, when the Visitor class isn't involved, because it is confusing. I think it's best to just refer to them as callbacks, or transformer callbacks.

[skepppy]

I reworded it again, removing the references to visitor.

[erezsh]

Thanks!

[nchammas]

FYI v_args also works for Interpreters.

[nchammas]

In addition to updating the docs, wouldn't it make sense to also have this decorator issue a runtime warning or even error out if used incorrectly on a Visitor?

[erezsh]

Yes, that sounds like the right thing to do.