docs: restore doc_cfg feature gate

[yuzheng14]

Problem

The documentation build for wreq v6.0.0-rc.29 fails on docs.rs with E0658 because #[doc(cfg)] is experimental:

• Failed docs.rs build for v6.0.0-rc.29

The package metadata passes --cfg docsrs to rustdoc. That activates the existing #[cfg_attr(docsrs, doc(cfg(...)))] annotations, but the crate-level doc_cfg feature gate is no longer present, so nightly rustdoc rejects those annotations.

Background

The conditional doc_cfg feature gate was removed in commit 29da5662, whose message describes doc_cfg as deprecated.

However, doc_cfg itself has not been deprecated or stabilized upstream:

• Rust tracking issue #43781 remains open and still identifies doc_cfg as the feature gate for #[doc(cfg)].
• The Rust 1.92 changes removed the separate doc_auto_cfg and doc_cfg_hide feature gates by merging them into doc_cfg; they did not remove doc_cfg itself.
• The doc_cfg stabilization PR #150055 is still open and unmerged.

Therefore, crates using explicit #[doc(cfg(...))] annotations still need to enable doc_cfg when docs.rs builds them with nightly Rust.

Fix

Restore the conditional crate attribute #![cfg_attr(docsrs, feature(doc_cfg))]. It is enabled only when cfg(docsrs) is set, so normal stable builds remain unaffected while docs.rs can build the feature availability annotations.

Verification

Both checks completed successfully locally:

• cargo package
  • packaged 88 files and successfully verified the packaged wreq source
• cargo +nightly rustdoc --lib --all-features -- --cfg docsrs
  • generated target/doc/wreq/index.html successfully