psa: Expand documentation for psa_key_agreement()
psa: Expand documentation for psa_key_agreement()

Document `peer_key` parameter requirements, including an explanation of
how the peer key is used and an example for EC keys.

Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls