chore: Replace doc_auto_cfg with doc_cfg

[sorairolake]

• Added a CHANGELOG.md entry

Summary

Replace the removed doc_auto_cfg feature with the doc_cfg feature.

Motivation

Avoid compilation errors when building documentation.

Details

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 errors on the latest nightlies:

error[E0557]: feature has been removed
  --> src/lib.rs:56:29
   |
56 | #![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`

error[E0557]: feature has been removed
  --> rand_core/src/lib.rs:36:29
   |
36 | #![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.

[josephlr]

Thanks, we had to do a similar thing in getrandom:

rust-random/getrandom@c904801