MD Settings: Add comment for sync page CSS rule

chrome/browser/resources/settings/people_page/sync_page.html

 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/web_ui_listener_behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-checkbox/paper-checkbox.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-input/paper-input.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-radio-button/paper-radio-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-radio-group/paper-radio-group.html">
 <link rel="import" href="/people_page/sync_browser_proxy.html">
 <link rel="import" href="/route.html">
@@ skipping 27 matching lines @@
 
       #existingPassphraseInput {
         /* The submit button for the existing passphrase is on the same line. */
         -webkit-margin-end: 16px;
         display: inline-block;
         --paper-input-container: {
           padding: 0;
         };
       }
 
+      /**
+       * This is currently necessary because a link inside a disabled
+       * paper-checkbox inherits `pointer-events: none;` and will not work.
+       * See: https://github.com/PolymerElements/paper-checkbox/issues/166
+       */
       #paymentLearnMore {
         pointer-events: initial;
       }
     </style>
     <div id="[[pages.SPINNER]]" class="settings-box first"
         hidden$="[[!isStatus_(pages.SPINNER, pageStatus_)]]">
       $i18n{syncLoading}
     </div>
     <div id="[[pages.TIMEOUT]]" class="settings-box first"
         hidden$="[[!isStatus_(pages.TIMEOUT, pageStatus_)]]">
@@ skipping 184 matching lines @@
           </div>
           <div id="passphraseRecoverHint" class="list-item">
             <span>$i18nRaw{passphraseRecover}</span>
           </div>
         </div>
       </template>
     </div>
   </template>
   <script src="sync_page.js"></script>
 </dom-module>