Run Documentation formatting in parallel

AsciiDoc is slow. Run 8 forks in parallel, assuming most developers
building Gerrit need at least 4 cores to get through the GWT builds.

Change-Id: Icf1a244d528f91aa5b8fbc10aa905ce3b32a3535
diff --git a/gerrit-war/pom.xml b/gerrit-war/pom.xml
index b359079..3702d98 100644
--- a/gerrit-war/pom.xml
+++ b/gerrit-war/pom.xml
@@ -251,6 +251,7 @@
                   <arg value="VERSION=${project.version}" />
                   <arg value="clean" />
                   <arg value="all" />
+                  <arg value="-j8" />
                 </exec>
 
                 <mkdir dir="${dst}" />
@@ -278,6 +279,7 @@
                   <arg value="VERSION=${project.version}" />
                   <arg value="clean" />
                   <arg value="all" />
+                  <arg value="-j8" />
                 </exec>
 
                 <mkdir dir="${dst}" />