Clarify output requirements
* - Bignum parameters called \p A, \p B, ... are inputs and are not
* modified by the function.
* - Bignum parameters called \p X, \p Y, ... are outputs or input-output.
- * The initial content of output-only parameters is ignored. These will
- * have the type #mbedtls_mpi_mod_residue.
+ * The initial bignum value of output-only parameters is ignored, but
+ * they must be set up and associated with the modulus \p N.
* - Bignum parameters called \p P are inputs used to setup a modulus or
* residue. These must be pointers to an array of limbs.
* - \p T is a temporary storage area. The initial content of such
