doc: document ios build tag for crypto/x509

Change-Id: I6e6d38ae347b4f5a33dff609b89785a038bc384c
Reviewed-on: https://go-review.googlesource.com/12304Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/go1.5.html

@@ -242,6 +242,11 @@ On NaCl, Go 1.5 requires SDK version pepper-39 or above because it now uses the
 <code>get_random_bytes</code> system call.
 </p>
 
+<p>
+On Darwin, the use of the system X.509 certificate interface can be disabled
+with the <code>ios</code> build tag.
+</p>
+
 <h2 id="tools">Tools</h2>
 
 <h3 id="translate">Translating</h3>