Document existing dependency of MD_C

Trying to compile MD_C without any of the hash modules would result in a
bunch of unused parameter warning (hence errors in -Werror builds).

We could silence those warnings by casting the parameters to void, but
still, compiling the module in such a configuration would mean all of
its functions are useless (always returning an error).

Seems better to just document the dependency.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

include/mbedtls/mbedtls_config.h

@@ -2423,6 +2423,9 @@
  *
  * Enable the generic message digest layer.
  *
+ * Requires: one of: MBEDTLS_MD5_C, MBEDTLS_RIPEMD160_C, MBEDTLS_SHA1_C,
+ *                   MBEDTLS_SHA224_C, MBEDTLS_SHA256_C, MBEDTLS_SHA384_C,
+ *                   MBEDTLS_SHA512_C.
  * Module:  library/md.c
  * Caller:  library/constant_time.c
  *          library/ecdsa.c