evp_rand: documentation

EVP_RAND, the RNGs and provider-rand.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pul

evp_rand: documentation

EVP_RAND, the RNGs and provider-rand.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/11682)

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