doc: NaCl requires pepper 41

Fixes #12062
Updates #11961

The sRPC nameservice was removed in pepper 42. For Go 1.5 stipulate
that NaCl requires pepper 41 only.

Change-Id: Ic88ba342d41f673391efaa96fb581712fa10a0fd
Reviewed-on: https://go-review.googlesource.com/13341Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/go1.5.html

@@ -247,8 +247,8 @@ On FreeBSD, Go 1.5 requires FreeBSD 8-STABLE+ because of its new use of the <cod
 </p>
 
 <p>
-On NaCl, Go 1.5 requires SDK version pepper-39 or above because it now uses the
-<code>get_random_bytes</code> system call.
+On NaCl, Go 1.5 requires SDK version pepper-41. Later pepper versions are not
+compatible due to the removal of the sRPC subsystem from the NaCl runtime.
 </p>
 
 <p>