chore: Replace doc_auto_cfg with doc_cfg

[sorairolake]

doc_auto_cfg was merged into doc_cfg in rust-lang/rust#138907.

When docsrs is enabled, the old doc_auto_cfg results in the following error on the latest nightlies:

error[E0557]: feature has been removed
  --> src/lib.rs:30:29
   |
30 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
   |                             ^^^^^^^^^^^^ feature has been removed
   |
   = note: removed in CURRENT_RUSTC_VERSION; see <https://github.com/rust-lang/rust/pull/138907> for more information
   = note: merged into `doc_cfg`

The new doc_cfg retains the automatic cfg generation that doc_auto_cfg used to provide.