Update doc references to new names for std, extra, and std::libc
Reproduce text changes from @brson PR 7176 and fix a typo therein
auto merge of #7197 : rkbodenner/rust/doc-for-lib-move, r=brson

Was updating some code of mine to use the new `std`/`extra` library names, and noticed a place where docs for `std::libc` hadn't been updated. Then I updated some top-level docs for the new libraries' names, too.