Bug 255938: Document warn on close pref in Advanced Options, p=jwalden, r=rlk. Also fix typo (bug 256041)

browser/components/help/locale/en-US/prefs.xhtml

@@ -288,7 +288,7 @@
   <p>This will display the Change Action window, where you can choose to have the file type opened
     by an application or saved to disk. For example, if you view lots of media files on Web
     pages, you might want to specify that &brandShortName; always open media files in your media
-    player insted of asking where you want each media file to be saved.</p>
+    player instead of asking where you want each media file to be saved.</p>
 
   <ul>
     <li><em>Open them in the Default application:</em><br/>Select this option to open this file type
@@ -368,6 +368,12 @@
     Check this option to load the link in a foreground tab instead, which will show that tab
     directly.</p>
 
+  <p><em>Warn when closing multiple tabs</em><br/>When you close a window with
+    multiple tabs, &brandShortName; will ask you to confirm your choice.  This
+    prevents you from accidentally closing the whole window when you wish to
+    close only the current tab.  Uncheck this box to skip past the warning
+    and have &brandShortName; automatically close the window.</p>
+
   <p><em>Use autoscrolling</em><br/>Autoscrolling is a useful feature which allows you to scroll
     the page by just holding down the middle mouse button (usually the scroll wheel) and move the
     mouse up or down. Some people find this annoying so here's the option for it.</p>