Use doc_cfg (#[doc(cfg(feature = "..."))]) to show feature requirements

[kazk]

Show these in docs for derive and ws:

![feature(doc_cfg)] is nightly only, but docs.rs uses nightly by default.

From https://stackoverflow.com/a/61417700

# docs.rs-specific configuration
[package.metadata.docs.rs]
# document all features
all-features = true
# defines the configuration attribute `docsrs`
rustdoc-args = ["--cfg", "docsrs"]

// only enables the `doc_cfg` feature when
// the `docsrs` configuration attribute is defined
#[cfg_attr(docsrs, feature(doc_cfg))]

#[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
pub fn test() {}