librand: Revise crypto part of document

This patch adds document which explains when to use `OSRng` in
cryptographic context, and explains why we use `/dev/urandom` instead
of `/dev/random`.
auto merge of #13820 : klutzy/rust/urandom, r=alexcrichton

This patch adds document which explains when to use `OSRng` in
cryptographic context, and explains why we use `/dev/urandom` instead
of `/dev/random`.