doc: fix required OS X version inconsistency for binary downloads

Updates golang#16625

Change-Id: Icac6705828bd9b29379596ba64b34d922b9002c3
Reviewed-on: https://go-review.googlesource.com/25548
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/install.html

@@ -17,7 +17,7 @@ <h2 id="download">Download the Go distribution</h2>
 <p>
 <a href="https://golang.org/dl/" target="_blank">Official binary
 distributions</a> are available for the FreeBSD (release 8-STABLE and above),
-Linux, Mac OS X (10.7 and above), and Windows operating systems and
+Linux, Mac OS X (10.8 and above), and Windows operating systems and
 the 32-bit (<code>386</code>) and 64-bit (<code>amd64</code>) x86 processor
 architectures.
 </p>
@@ -49,7 +49,7 @@ <h2 id="requirements">System requirements</h2>
 <tr><td colspan="3"><hr></td></tr>
 <tr><td>FreeBSD 8-STABLE or later</td> <td>amd64</td> <td>Debian GNU/kFreeBSD not supported</td></tr>
 <tr><td>Linux 2.6.23 or later with glibc</td> <td>amd64, 386, arm</td> <td>CentOS/RHEL 5.x not supported</td></tr>
-<tr><td>Mac OS X 10.7 or later</td> <td>amd64</td> <td>use the clang or gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup></td></tr>
+<tr><td>Mac OS X 10.7 or later</td> <td>amd64</td> <td>use the clang or gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup> for <code>cgo</code> support</td></tr>
 <tr><td>Windows XP or later</td> <td>amd64, 386</td> <td>use MinGW gcc<sup>&#8224;</sup>. No need for cygwin or msys.</td></tr>
 </table>