Document const parameter conventions
* of type #mbedtls_mpi_mod_modulus. The structure must be set up with an
* array of limbs storing the bignum value of the modulus. The modulus must
* be odd and is assumed to have no leading zeroes. The modulus is usually
- * named \c N and is usually input-only.
+ * named \c N and is usually input-only. Functions which take a parameter
+ * of type \c const #mbedtls_mpi_mod_modulus* must not modify its value.
* - **Bignum parameters**: Bignums are passed as pointers to an array of
* limbs or to a #mbedtls_mpi_mod_residue structure. A limb has the type
* #mbedtls_mpi_uint. Residues must be initialized before use, and must be
* associated with the modulus \c N. Unless otherwise specified:
* - Bignum parameters called \c A, \c B, ... are inputs and are not
- * modified by the function.
+ * modified by the function. Functions which take a parameter of
+ * type \c const #mbedtls_mpi_mod_residue* must not modify its value.
* - Bignum parameters called \c X, \c Y, ... are outputs or input-output.
* The initial bignum value of output-only parameters is ignored, but
* they must be set up and associated with the modulus \c N. Some
