doc: update Mercurial installation instructions

R=r
CC=golang-dev
https://golang.org/cl/6031046

doc/install-source.html

@@ -106,26 +106,13 @@ On Windows, install <code>gcc</code> with
 
 <p>
 To perform the next step you must have Mercurial installed. (Check that you
-have an <code>hg</code> command.) This suffices to install Mercurial on most
-Unix systems:
+have an <code>hg</code> command.)
 </p>
 
-<pre>
-sudo easy_install mercurial==2.0
-</pre>
-
-<p>
-On Ubuntu/Debian, the Mercurial in your distribution's
-package repository is most likely old and broken.
-You might try this first:
-</p>
-
-<pre>apt-get install python-setuptools python-dev build-essential</pre>
-
 <p>
-If that fails, or if you use Windows, install manually from the
-<a href="http://mercurial.selenic.com/wiki/Download">Mercurial Download</a>
-page.
+If you do not have a working Mercurial installation,
+follow the instructions on the
+<a href="http://mercurial.selenic.com/downloads/">Mercurial downloads</a> page.
 </p>
 
 <p>