[release-branch.go1.3] doc: mention WriteHeapDump in 1.3 release notes

««« CL 103810044 / 603f6c3b152c
doc: mention WriteHeapDump in 1.3 release notes

LGTM=r
R=khr, r
CC=golang-codereviews
https://golang.org/cl/103810044
»»»

TBR=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/99700043

doc/go1.3.html

@@ -539,6 +539,12 @@ Earlier versions of Go handled this situation inconsistently: most instances
 were reported as deadlocks, but some trivial cases exited cleanly instead.
 </li>
 
+<li>
+The runtime/debug package now has a new function
+<a href="/pkg/runtime/debug/#WriteHeapDump"><code>debug.WriteHeapDump</code></a>
+that writes out a description of the heap.
+</li>
+
 <li>
 The <a href="/pkg/strconv/#CanBackquote"><code>CanBackquote</code></a>
 function in the <a href="/pkg/strconv/"><code>strconv</code></a> package