improve docs for writing attribute procedural macros that parse ItemFn

[zachreizner]

The signature of these procedural macros is fn cool_fn_macro(attr: TokenStream, item: TokenStream) -> TokenStream and they usually start with let mut function = parse_macro_input!(item as ItemFn);. The attr argument that the macro receives includes only the inner part of the attribute:

#[cool_fn_macro(category = "suite", name = "task_long")]
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                            attr: TokenStream

The way to parse this attr is with parse_macro_input!(attr as Vec<NestedMeta>) or equivalently parse_macro_input!(attr as AttributeArgs). But this was hard to discover on my part.

I suggest including this last bit of information in the AttributeArgs, Meta, and any related pages so that future authors will have an easier time than I did. In particular, it would be nice to find the correct type in the doc index by searching for similar syntax, similarly to how the short docs for the other types is written out.

[ultimaweapon]

Seems like this is not valid for v2.