Move implementation specific comment

This comment is about how the functions are implemented, not about their
public interface, so it doesn't belong in the header file.
It applies to everything in constant_time.c so moved there.

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
diff --git a/library/constant_time.c b/library/constant_time.c
index e0eadac..98fdbd7 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -17,6 +17,11 @@
  *  limitations under the License.
  */
 
+ /*
+ * The following functiona are implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
+ */
+
 #include "common.h"
 #include "constant_time.h"
 #include "mbedtls/error.h"