Correct RFC reference in time::Tm::rfc3339 docs

Fixes #16158
Fix API docs css reversing elements that it shouldn't

remove unneeded `pre.rust a' selector

move transform into `.test-arrow`

fixes #16138
auto merge of #16164 : anguslees/rust/rfc3999, r=alexcrichton

Fixes #16158
auto merge of #16165 : tinaun/rust/newcss, r=alexcrichton

remove unneeded `pre.rust a' selector

move transform into `.test-arrow`

fixes #16138