doc/go1.10: preannounce deprecation of OS X 10.9 Mavericks

By the time Go 1.11 is released, OS X 10.9 Mavericks will have gone
two years with no security updates.

For #23011.

Change-Id: I6482852a14477985769b72c45c92416aae8be100
Reviewed-on: https://go-review.googlesource.com/83795
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: rsc

doc/go1.10.html

@@ -65,8 +65,8 @@ <h2 id="ports">Ports</h2>
 </p>
 
 <p id="darwin">
-Go 1.10 is the last release that will run on OS X 10.8 Mountain Lion.
-Go 1.11 will require OS X 10.9 Mavericks or later.
+Go 1.10 is the last release that will run on OS X 10.8 Mountain Lion or on OS X 10.9 Mavericks.
+Go 1.11 will require OS X 10.10 Yosemite or later.
 </p>
 
 <p id="freebsd">
@@ -76,7 +76,7 @@ <h2 id="ports">Ports</h2>
 </p>
 
 <p id="netbsd">
-Go now runs on NetBSD again, but requires the unreleased NetBSD 8.
+Go now runs on NetBSD again but requires the unreleased NetBSD 8.
 Only <code>GOARCH</code> <code>amd64</code> and <code>386</code> are
 known to work. The <code>arm</code> port might work but is untested.
 </p>