doc/go1.7: document signal name printing during panics

Document new behavior about signal name printing
in panics as per CL golang.org/cl/22753.

For #15810

Change-Id: I9c677d5dd779b41e82afa25e3c797d8e739600d3
Reviewed-on: https://go-review.googlesource.com/23493Reviewed-by: Russ Cox <rsc@golang.org>

doc/go1.7.html

@@ -397,6 +397,11 @@ as
 <a href="/ref/spec#Run_time_panics">required by the language specification</a>.
 </p>
 
+<p>
+During panics, if a signal's name is known, it will be printed in the stack trace.
+Otherwise, the signal's number will be used, as it was before Go1.7.
+</p>
+
 <p>
 The new function
 <a href="/pkg/runtime/#KeepAlive"><code>KeepAlive</code></a>