doc: give up on OS X Leopard

Fixes #3206.

R=golang-dev, gri, bradfitz
CC=golang-dev
https://golang.org/cl/5769046

doc/install-source.html

@@ -321,7 +321,7 @@ These default to the values of <code>$GOHOSTOS</code> and
 
 <p>
 Choices for <code>$GOOS</code> are
-<code>darwin</code> (Mac OS X 10.5 and above), <code>freebsd</code>,
+<code>darwin</code> (Mac OS X 10.6 and above), <code>freebsd</code>,
 <code>linux</code>, <code>netbsd</code>, <code>openbsd</code>, 
 <code>plan9</code>, and <code>windows</code>.
 Choices for <code>$GOARCH</code> are

doc/install.html

@@ -35,7 +35,7 @@ your operating system and processor architecture.
 
 <p>
 Official binary distributions are available
-for the FreeBSD, Linux, Mac OS X, and Windows operating systems
+for the FreeBSD, Linux, Mac OS X (Snow Leopard/Lion), and Windows operating systems
 and the 32-bit (<code>386</code>) and 64-bit (<code>amd64</code>)
 x86 processor architectures.
 </p>