doc: decrease prominence of GOROOT_BOOTSTRAP

Go build scripts on UNIX (make.bash, all.bash) have not required
GOROOT_BOOTSTRAP since August 2017 (CL 57753). Windows build scripts
have followed suit since CL 96455. Most people building Go will have
a Go toolchain in their PATH and will not need to specify a different
toolchain.

This CL removes the GOROOT_BOOTSTRAP mention from the contribution guide
(it was there for Windows only, but it's not required anymore). The guide
is meant to be light and clear for beginners and is not supposed to be
a reference, so there's not need to keep mentioning GOROOT_BOOTSTRAP.

Also update install-source.html to reflect the current status quo,
where using the PATH is probably the first and most used default, and
GOROOT_BOOTSTRAP is just an option.

Change-Id: Iab453e61b0c749c256aaaf81ea9b2ae58822cb89
Reviewed-on: https://go-review.googlesource.com/c/go/+/224717
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/doc/contribute.html b/doc/contribute.html
index 551d510..4135d13 100644
--- a/doc/contribute.html
+++ b/doc/contribute.html
@@ -552,9 +552,7 @@
 </pre>
 
 <p>
-(To build under Windows use <code>all.bat</code>; this also requires
-setting the environment variable <code>GOROOT_BOOTSTRAP</code> to the
-directory holding the Go tree for the bootstrap compiler.)
+(To build under Windows use <code>all.bat</code>)
 </p>
 
 <p>
diff --git a/doc/install-source.html b/doc/install-source.html
index 17b1c9c..3d42a10 100644
--- a/doc/install-source.html
+++ b/doc/install-source.html
@@ -106,23 +106,17 @@
 
 </div>
 
-<h2 id="go14">Install Go compiler binaries</h2>
+<h2 id="go14">Install Go compiler binaries for bootstrap</h2>
 
 <p>
 The Go toolchain is written in Go. To build it, you need a Go compiler installed.
-The scripts that do the initial build of the tools look for an existing Go tool
-chain in <code>$GOROOT_BOOTSTRAP</code>.
-If unset, the default value of <code>GOROOT_BOOTSTRAP</code>
-is <code>$HOME/go1.4</code>.
-</p>
-
-<p>
-There are many options for the bootstrap toolchain.
-After obtaining one, set <code>GOROOT_BOOTSTRAP</code> to the
-directory containing the unpacked tree.
-For example, <code>$GOROOT_BOOTSTRAP/bin/go</code> should be
-the <code>go</code> command binary for the bootstrap toolchain.
-</p>
+The scripts that do the initial build of the tools look for a "go" command
+in <code>$PATH</code>, so as long as you have Go installed in your
+system and configured in your <code>$PATH</code>, you are ready to build Go
+from source. 
+Or if you prefer you can set <code>$GOROOT_BOOTSTRAP</code> to the
+root of a Go installation to use to build the new Go toolchain;
+<code>$GOROOT_BOOTSTRAP/bin/go</code> should be the go command to use.</p>
 
 <h3 id="bootstrapFromBinaryRelease">Bootstrap toolchain from binary release</h3>