Clarify floating-point to integral conversion rounding behaviour

generators/builtins/primitives.kt

@@ -241,7 +241,7 @@ class GeneratePrimitives(out: PrintWriter) : BuiltInsSourceGenerator(out) {
             """
         } else {
             """
-             * The fractional part, if any, is rounded down.
+             * The fractional part, if any, is rounded down towards zero.
              * Returns zero if this `$thisName` value is `NaN`, [$otherName.MIN_VALUE] if it's less than `$otherName.MIN_VALUE`,
              * [$otherName.MAX_VALUE] if it's bigger than `$otherName.MAX_VALUE`.
              */

generators/builtins/unsignedTypes.kt

@@ -349,7 +349,7 @@ class UnsignedTypeGenerator(val type: UnsignedType, out: PrintWriter) : BuiltIns
                 /**
                  * Converts this [$otherName] value to [$className].
                  *
-                 * The fractional part, if any, is rounded down.
+                 * The fractional part, if any, is rounded down towards zero.
                  * Returns zero if this `$otherName` value is negative or `NaN`, [$className.MAX_VALUE] if it's bigger than `$className.MAX_VALUE`.
                  */
                 """.trimIndent()