math: add doc note about floating point operation

Go doesn't guarantee that the result of floating point operations will
be the same on different architectures. It was not stated in the
documentation, that can lead to confusion.

Fixes #18354

Change-Id: Idb1b4c256fb9a7158a74256136eca3b8ce44476f
Reviewed-on: https://go-review.googlesource.com/34938Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/math/const.go

@@ -3,6 +3,9 @@
 // license that can be found in the LICENSE file.
 
 // Package math provides basic constants and mathematical functions.
+//
+// Note that the package doesn't guarantee identical results of floating point operations on
+// different architectures.
 package math
 
 // Mathematical constants.