doc: mention signal changes for c-archive/c-shared

Change-Id: Ibba7fccba9617612e026bd0a208eb12918de465a
Reviewed-on: https://go-review.googlesource.com/18985Reviewed-by: Russ Cox <rsc@golang.org>

doc/go1.6.html

@@ -341,6 +341,19 @@ and changing the global timer resolution caused problems on some systems,
 so the call has been removed.
 </p>
 
+<p>
+When using <code>-buildmode=c-archive</code> or
+<code>-buildmode=c-shared</code> to build an archive or a shared
+library, the handling of signals has changed.
+In Go 1.5 the archive or shared library would install a signal handler
+for most signals.
+In Go 1.6 it will only install a signal handler for the
+synchronous signals needed to handle run-time panics in Go code:
+SIGBUS, SIGFPE, SIGSEGV.
+See the <a href="/pkg/os/signal">os/signal</a> package for more
+details.
+</p>
+
 <h3 id="reflect">Reflect</h3>
 
 <p>