Add commentary about lock usage in provider_core.c

Add commentary about lock usage in provider_core.c

Provide some guidelines, as well as some rules for using the locks in
provider_core.c, in order to avoid the introduction of deadlocks.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16469)