Add /doc/{flate,rustdoc} to .gitignore
Minor fix to docs.
Feature gate #[simd]

Fixes #11721
auto merge of #11738 : dmanescu/rust/11721, r=alexcrichton

Fixes #11721
auto merge of #11833 : kballard/rust/gitignore-doc-flate-rustdoc, r=brson
auto merge of #11837 : xales/rust/doc-fixes, r=cmr,huonw,alexcrichton