Document UTF-8 expectation for pass phrases passed to OSSL_STORE

After some discussion, it was concluded that the better idea is to
stipulate that the pass phrases passed to the OSSL_STORE API are
expected to be UTF-8 encoded, and that all objects made accessible
through OSSL_STORE URIs should adhere to this expectation (at the
discretion of the loaders).

Email ref:
https://mta.openssl.org/pipermail/openssl-project/2018-June/000771.html



Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6416)