Clarify mpi_gdc() documentation when B is 0
Signed-off-by: Felix Conway <felix.conway@arm.com>
diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index b06aec9..297c0a6 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -974,6 +974,8 @@
* \brief Compute the greatest common divisor: G = gcd(A, B)
*
* \param G The destination MPI. This must point to an initialized MPI.
+ * This will be positive unless \p B is 0, in which case \p A
+ * will be returned, where \p A could be negative.
* \param A The first operand. This must point to an initialized MPI.
* \param B The second operand. This must point to an initialized MPI.
*