Make readonly fields visible in rustdoc

[dtolnay]

By making them pub in the case of #[cfg(rustdoc)].

For example with the invocation from the readme:

#[readonly::make]
pub struct S {
    pub n: i32,
}

we will need to generate:

  #[repr(C)]
  pub struct S {
+     #[cfg(not(rustdoc))]
      n: i32,
+     #[cfg(rustdoc)]
+     pub n: i32,
  }

  #[repr(C)]
  pub struct ReadOnlyS {
      pub n: i32,
  }

  impl core::ops::Deref for S {
      type Target = ReadOnlyS;
      ...
  }

Currently blocked on rust-lang/rust#43781.

[dtolnay]

Fixed in 11457e3 by requiring opt-in at the call site.

#[readonly::make(doc = mycratename_doc_cfg)]
pub struct MyStruct {...}

and in Cargo.toml:

[package.metadata.docs.rs]
rustdoc-args = ["--cfg", "mycratename_doc_cfg"]