`rustdoc-args` is ignored for proc_macro crates

Crate name

attr_alias

Build failure link

https://docs.rs/crate/attr_alias/0.1.0/builds/1075851

Additional details

The build did not fail, but this still appeared to be the best template, as the build did not work as expected.

For proc_macro crates, docs.rs avoids passing --target for builds:
https://github.com/rust-lang/docs.rs/blob/b0f6feadba64502c8aa68dedc3e20be096c5af23/src/docbuilder/rustwide_builder.rs#L830-L833

From what I can tell, for library crates, this fixes an issue of rustdoc-args being ignored when passed as build.rustdocflags to rustdoc. However, regardless of whether --target is passed, rustdoc-args is ignored for proc_macro crates.

For example, Cargo.toml for attr_alias includes:

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

attr_alias::attr_alias also has the attribute:

#[cfg_attr(attr_alias_docs_rs, doc(cfg(feature = "nightly")))]

Therefore, attr_alias::attr_alias should be documented to require the "nightly" feature, but the note is missing, since rustdoc-args was ignored:
https://docs.rs/attr_alias/0.1.0/attr_alias/attr.attr_alias.html

Start in src/docbuilder/rustwide_builder.rs around lines 830-833 and trace the proc_macro build path where --target is omitted. Reproduce the attr_alias 0.1.0 case using its docs.rs configuration, then verify that rustdoc-args reaches rustdoc and the generated documentation shows the nightly feature requirement.