improve docs for writing attribute procedural macros that parse ItemFn

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)` 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](https://docs.rs/syn/0.15.30/syn/index.html) by searching for similar syntax, similarly to how the short docs for the other types is written out.