doc: in Go 1.8 notes, mention lack of RWMutex in contention profile

For #18496.

Change-Id: I50ced7c9f0fe5d9c627eef1f59a7f73be742e04c
Reviewed-on: https://go-review.googlesource.com/34831
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: rsc

doc/go1.8.html

@@ -631,6 +631,13 @@ <h3 id="mutex_prof">Mutex Contention Profiling</h3>
   <a href="/pkg/runtime/#SetMutexProfileFraction"><code>SetMutexProfileFraction</code></a>.
 </p>
 
+<p>
+  A known limitation for Go 1.8 is that the profile only reports contention for
+  <a href="/pkg/sync/#Mutex"><code>sync.Mutex</code></a>,
+  not 
+  <a href="/pkg/sync/#RWMutex"><code>sync.RWMutex</code></a>.
+</p>
+
 <h3 id="minor_library_changes">Minor changes to the library</h3>
 
 <p>