Options: add a "Learn more" link to Android Apps section and tweak strings

chrome/browser/resources/options/browser_options.html

 <div id="settings" class="page" hidden>
   <header>
     <h1 i18n-content="settingsTitle"></h1>
   </header>
   <include src="automatic_settings_reset_banner.html">
 <if expr="chromeos">
   <link rel="import" href="chrome://resources/cr_elements/network/cr_network_icon.html">
   <link rel="import" href="chrome://resources/cr_elements/network/cr_onc_types.html">
   <include src="secondary_user_banner.html">
   <section id="network-section-cros">
@@ skipping 219 matching lines @@
     <div class="checkbox controlled-setting-with-label">
       <label>
         <input id="android-apps-enabled" pref="arc.enabled"
             metric="Options_AndroidApps" type="checkbox" dialog-pref>
         <span>
           <span i18n-content="androidAppsEnabled"></span>
           <span class="controlled-setting-indicator"
                 pref="arc.enabled"></span>
         </span>
       </label>
+      <a target="_blank" i18n-content="learnMore"
+          href="http://support.google.com/chromebook?p=playapps"></a>
     </div>
     <div id="android-apps-settings" class="controlled-setting-with-label" hidden>
       <label>
         <span id="android-apps-settings-label"></span>
       </label>
     </div>
   </section>
 </if>
   <section id="sync-users-section" guest-visibility="hidden">
     <h3 i18n-content="sectionTitleUsers"></h3>
@@ skipping 870 matching lines @@
       </button>
     </div>
   </section>
   </div>  <!-- advanced-settings-container -->
   </div>  <!-- advanced-settings -->
   <footer id="advanced-settings-footer">
     <a is="action-link" id="advanced-settings-expander"
         i18n-content="showAdvancedSettings"></a>
   </footer>
 </div>