Missing documentation for rustdoc features `doc_cfg_hide` and `doc_auto_cfg`

[Nemo157]

#89596 and #90502 added two new sub-features of doc_cfg but these are currently undocumented outside the implementation. They should probably both be sub-headings under https://doc.rust-lang.org/nightly/rustdoc/unstable-features.html#doccfg-recording-what-platforms-or-features-are-required-for-code-to-be-present.