Add a warning about locking in the child provider callback docs

Add a warning about locking in the child provider callback docs

The child provider callbacks can hold the store lock. In order to avoid
deadlocks we require that the callback implementations don't themselves
call functions that may aquire those locks.

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