math: clarify comment about bit-identical results across architectures

Updates #18354.

Change-Id: I76bc4a73d8dc99eeda14b395e451d75a65184191
Reviewed-on: https://go-review.googlesource.com/45013Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/math/const.go

@@ -4,8 +4,7 @@
 
 // Package math provides basic constants and mathematical functions.
 //
-// Note that the package doesn't guarantee identical results of floating point operations on
-// different architectures.
+// This package does not guarantee bit-identical results across architectures.
 package math
 
 // Mathematical constants.