doc: document that 'openssl rand' is cryptographically secure

doc: document that 'openssl rand' is cryptographically secure

(cherry picked from commit 88398d2a358f)

Additionally, remove an outdated paragraph mentioning the .rnd
file, which is obsolete in 1.1.1 since the RANDFILE entry was
removed from openssl.cnf in commit 1fd6afb571e8.

Also borrow some text from 'openssl(1)/Random State Options'
on master (commit a397aca43598) to emphasize that it is not
necessary anymore to restore and save the RNG state using the
'-rand' and '-writerand' options.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11251)