Update dhm.h
Update dhm.h

Minor documentation improvements:
*Standardized file brief description.
*Separated return statements.
*Reordered tags within documentation blocks so that params and returns are last in block.
*p_rng descriptions changed from "parameter" to "context".
*Suggest to specify issue for each return code, where multiple failure return codes are listed, or generalize.
*Minor improvements to parameter documentation proposed by eng.
ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls