Use official cfg(docsrs) for doc_cfg

This reverts 3ad5d30.

Author: taiki-e

.github/workflows/ci.yml

@@ -51,8 +51,6 @@ jobs:
       event_name: ${{ github.event_name }}
   tidy:
     uses: taiki-e/github-actions/.github/workflows/tidy.yml@main
-    with:
-      known-cfgs: syn_serde_doc_cfg
 
   test:
     strategy:

Cargo.toml

@@ -14,7 +14,6 @@ Library to serialize and deserialize Syn syntax trees.
 
 [package.metadata.docs.rs]
 all-features = true
-rustdoc-args = ["--cfg", "syn_serde_doc_cfg"]
 targets = ["x86_64-unknown-linux-gnu"]
 
 [package.metadata.cargo_check_external_types]

src/lib.rs

@@ -128,8 +128,8 @@ The data structures of syn-serde 0.3 is compatible with the data structures of
     clippy::used_underscore_binding,
     clippy::wildcard_imports
 )]
-// docs.rs only (cfg is enabled via [package.metadata.docs.rs] in Cargo.toml, not build script)
-#![cfg_attr(syn_serde_doc_cfg, feature(doc_cfg))]
+// docs.rs only (cfg is enabled by docs.rs, not build script)
+#![cfg_attr(docsrs, feature(doc_cfg))]
 
 #[macro_use]
 mod macros;
@@ -259,7 +259,7 @@ pub use crate::token_stream::{
 };
 
 #[cfg(feature = "json")]
-#[cfg_attr(syn_serde_doc_cfg, doc(cfg(feature = "json")))]
+#[cfg_attr(docsrs, doc(cfg(feature = "json")))]
 pub mod json;
 
 mod sealed {