Fix up documentation around no_std
1. Fix the sections in the book to have the correct signatures. I've
also marked them as `ignore`; there's no way to set the `no_std`
feature for libc, so it pulls in the stdlib, so this wasn't even
testing the actual thing it was testing. Better to just ignore.
2. Correcting libcore's docs for factual inaccuracy, and add a note
about language items.
Fixes#33677
r? @alexcrichton
689fde2735