doc: mention the install locations for godoc, cover, and vet

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13893043

doc/install-source.html

@@ -250,6 +250,16 @@ You must also have a workspace (<code>GOPATH</code>) set up;
 see <a href="/doc/code.html">How to Write Go Code</a> for the details.
 </p>
 
+<p>
+<b>Note</b>: The <code>go</code> command will install the <code>godoc</code>
+binary to <code>$GOROOT/bin</code> (or <code>$GOBIN</code>) and the
+<code>cover</code> and <code>vet</code> binaries to
+<code>$GOROOT/pkg/tool/$GOOS_$GOARCH</code>.
+You can access the latter commands with
+"<code>go</code> <code>tool</code> <code>cover</code>" and
+"<code>go</code> <code>tool</code> <code>vet</code>".
+</p>
+
 <h2 id="community">Community resources</h2>
 
 <p>