obsolete.rs: fix typo in message
rustc.1: fix typo

Uses the same wording as ``config.rs`` now.
complement-lang-faq.md: fix typos
Examples for Integer trait methods.
Main example for bigint usage.

Also use `///` for documentation instead of `/**`. End comments with
`.`.

Show imports for bigint example.

They might be useful.
auto merge of #15706 : phi-gamma/rust/master, r=huonw

I kept changes to each file in a separate commit. Please let me know if you prefer them
squashed!
auto merge of #15716 : treeman/rust/bigint-doc, r=alexcrichton

Also use `///` for documentation instead of `/**`. End comments with `.`.
auto merge of #15718 : treeman/rust/integer-doc, r=alexcrichton

Simple usage examples for Integer methods. Also group `div_rem` and `div_mod_floor` together at the bottom of the trait, to reflect the documentation rendering.