Improve documentation of mbedtls_psa_inject_entropy
Improve documentation of mbedtls_psa_inject_entropy

Explain what the function does, why one would use it, how to use it,
how to handle its input, and what the status codes mean.

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