Clarify the documentation of mbedtls_pk_setup_opaque

[gilles-peskine-arm]

In the documentation of mbedtls_pk_setup_opaque, clarify which algorithm the functions mbedtls_pk_{sign,verify,encrypt,decrypt} will perform with the resulting key.

Definition of done: updated documentation, and possibly add unit tests if the documentation ends up stating things that are currently untested.

Related, but should in principle be independent: https://github.com/Mbed-TLS/mbedtls/issues/8710

[mpg]

Note: adding this to the "legacy-to-PSA migration helpers" EPIC as I think that was the intention (was on the EPICs board but not in any EPIC).