Remove incorrect constant-time claim from doc for mbedtls_mpi_core_add_if()

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index e443d2c..cf7caee 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -236,8 +236,8 @@
                                        size_t limbs );
 
 /**
- * \brief Constant-time conditional addition of two known-size large unsigned
- *        integers, returning the carry.
+ * \brief Conditional addition of two known-size large unsigned integers,
+ *        returning the carry.
  *
  * Functionally equivalent to
  *