Only deny doc_keyword in std and set it as "allow" by default

Author: GuillaumeGomez

std/src/lib.rs

@@ -212,6 +212,7 @@
     all(target_vendor = "fortanix", target_env = "sgx"),
     feature(slice_index_methods, coerce_unsized, sgx_platform)
 )]
+#![deny(rustc::existing_doc_keyword)]
 #![cfg_attr(all(test, target_vendor = "fortanix", target_env = "sgx"), feature(fixed_size_array))]
 // std is implemented with unstable features, many of which are internal
 // compiler details that will never be stable