Document attribute doc and its sub-language

[pnkfelix]

Sub-bug of #6444. Almost certainly ties into documenting rustdoc itself.

[flaper87]

triage bump, still missing

[burdges]

It appears that doc comments placed inside macro_rules! blocks do not get macro substitutions.

In this example, we need the #[doc(hidden)] or else the doc comments do nothing at all, but even with that the doc comment contains an unsubstituted $f.

#[doc(hidden)]
macro_rules! my_wrapper{
    ($n:ident,$f:ident) =>{

/// Wrapper that calls `$f` before doing ...
struct $n<T> where T: ...;

impl<T> $n<T> where T: ... {
    ... make something call $f ...
}

    }
}

Is there a way convert identifiers into strings and concatenate them in attributes? Appears that stringify! does not work in an attribute.