doc: document new linker -X syntax in go1.5.html

Fixes #11973.

Change-Id: Icffa3213246663982b7cc795982e0923e272f405
Reviewed-on: https://go-review.googlesource.com/12919
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.5.html

@@ -416,8 +416,8 @@ by the environment variables <code>GOOS</code> and <code>GOARCH</code>.
 </p>
 
 <p>
-There are a couple of other changes.
-The more significant is the addition of a <code>-buildmode</code> option that
+There are several other changes.
+The most significant is the addition of a <code>-buildmode</code> option that
 expands the style of linking; it now supports
 situations such as building shared libraries and allowing other languages
 to call into Go libraries.
@@ -436,6 +436,29 @@ Also, although this may be fixed, Windows cgo executables are missing some
 DWARF information.
 </p>
 
+<p>
+Finally, the <code>-X</code> flag, which takes two arguments,
+as in
+</p>
+
+<pre>
+-X importpath.name value
+</pre>
+
+<p>
+now also accepts a more common Go flag style with a single argument
+that is itself a <code>name=value</code> pair:
+</p>
+
+<pre>
+-X importpath.name=value
+</pre>
+
+<p>
+Although the old syntax still works, it is recommended that uses of this
+flag in scripts and the like be updated to the new form.
+</p>
+
 <h3 id="go_command">Go command</h3>
 
 <p>