Clarify how Quotient tests do rounding.

Change-Id: Ifea17a894065cce607845208c96e2092e4632d61
Reviewed-on: https://boringssl-review.googlesource.com/8607
Reviewed-by: Adam Langley <agl@google.com>

crypto/bn/bn_tests.txt

@@ -5957,7 +5957,8 @@ B = -542fb814f45924aa09a16f2a6
 
 # Quotient tests.
 #
-# These test vectors satisfy Quotient = A / B and Remainder = A - B * Quotient.
+# These test vectors satisfy Quotient = A / B, rounded towards zero, and
+# Remainder = A - B * Quotient.
 
 Quotient = 1
 Remainder = 0