doc: replace absolute links to golang.org with relative links

Currently tip.golang.org leads to golang.org and
local godoc also leads to golang.org (when you don't have internet connectivity).

LGTM=crawshaw
R=golang-codereviews, crawshaw
CC=golang-codereviews
https://golang.org/cl/100200043

doc/effective_go.html

@@ -214,7 +214,7 @@ not be used.
 One adjustment <code>godoc</code> does do is to display indented
 text in a fixed-width font, suitable for program snippets.
 The package comment for the
-<a href="http://golang.org/pkg/fmt/"><code>fmt</code> package</a> uses this to good effect.
+<a href="/pkg/fmt/"><code>fmt</code> package</a> uses this to good effect.
 </p>
 
 <p>
@@ -710,7 +710,7 @@ Erroneous encodings consume one byte and produce the
 replacement rune U+FFFD.
 (The name (with associated builtin type) <code>rune</code> is Go terminology for a
 single Unicode code point.
-See <a href="http://golang.org/ref/spec#Rune_literals">the language specification</a>
+See <a href="/ref/spec#Rune_literals">the language specification</a>
 for details.)
 The loop
 </p>

doc/go1.3.html

@@ -233,7 +233,7 @@ The cumulative effect can be a 50-70% reduction in collector pause time.
 </li>
 
 <li>
-The race detector (see <a href="http://golang.org/doc/articles/race_detector.html">this guide</a>)
+The race detector (see <a href="/doc/articles/race_detector.html">this guide</a>)
 is now about 40% faster.
 </li>
 

doc/go_faq.html

@@ -940,9 +940,9 @@ How are libraries documented?</h3>
 There is a program, <code>godoc</code>, written in Go, that extracts
 package documentation from the source code. It can be used on the
 command line or on the web. An instance is running at
-<a href="http://golang.org/pkg/">http://golang.org/pkg/</a>.
+<a href="/pkg/">http://golang.org/pkg/</a>.
 In fact, <code>godoc</code> implements the full site at
-<a href="http://golang.org/">http://golang.org/</a>.
+<a href="/">http://golang.org/</a>.
 </p>
 
 <h3 id="Is_there_a_Go_programming_style_guide">
@@ -1437,7 +1437,7 @@ each closure shares that single variable. When the closure runs, it prints the
 value of <code>v</code> at the time <code>fmt.Println</code> is executed,
 but <code>v</code> may have been modified since the goroutine was launched.
 To help detect this and other problems before they happen, run
-<a href="http://golang.org/cmd/go/#hdr-Run_go_tool_vet_on_packages"><code>go vet</code></a>.
+<a href="/cmd/go/#hdr-Run_go_tool_vet_on_packages"><code>go vet</code></a>.
 </p>
 
 <p>

doc/go_spec.html

@@ -23,7 +23,7 @@ TODO
 
 <p>
 This is a reference manual for the Go programming language. For
-more information and other documents, see <a href="http://golang.org/">http://golang.org</a>.
+more information and other documents, see <a href="/">http://golang.org</a>.
 </p>
 
 <p>