Simplify code

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 117c3c5..397f79a 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -85,8 +85,10 @@
  * most significant zero bytes in the input).
  *
  * \param X      The address of the MPI.
+ *               May only be #NULL if \nx is 0 and \p buflen is 0.
  * \param nx     The number of limbs of \p X.
  * \param buf    The input buffer to import from.
+ *               May only be #NULL if \p buflen is 0.
  * \param buflen The length in bytes of \p buf.
  *
  * \return       \c 0 if successful.