[fix] Fix format of settings template + add information about shortcuts

There was an extra comma at the end of shortcut list, this was breaking
Etherpad startup.

This is an adjustment to #2891.

settings.json.template

@@ -75,26 +75,26 @@
 
   /* Pad Shortcut Keys */
   "padShortcutEnabled" : {
-    "altF9" : true,
-    "altC" : true,
-    "cmdShift2" : true,
+    "altF9"     : true, /* focus on the File Menu and/or editbar */
+    "altC"      : true, /* focus on the Chat window */
+    "cmdShift2" : true, /* shows a gritter popup showing a line author */
     "delete"    : true,
     "return"    : true,
-    "cmdS" : true,
-    "tab" : true,
-    "cmdZ" : true,
-    "cmdY" : true,
-    "cmdI" : true,
-    "cmdB" : true,
-    "cmdU" : true,
-    "cmd5" : true,
-    "cmdShiftL" : true,
-    "cmdShiftN" : true,
-    "cmdShiftC" : true,
-    "cmdH" : true,
-    "ctrlHome" : true,
+    "cmdS"      : true, /* save a revision */
+    "tab"       : true, /* indent */
+    "cmdZ"      : true, /* undo/redo */
+    "cmdY"      : true, /* redo */
+    "cmdI"      : true, /* italic */
+    "cmdB"      : true, /* bold */
+    "cmdU"      : true, /* underline */
+    "cmd5"      : true, /* strike through */
+    "cmdShiftL" : true, /* unordered list */
+    "cmdShiftN" : true, /* ordered list */
+    "cmdShiftC" : true, /* clear authorship */
+    "cmdH"      : true, /* backspace */
+    "ctrlHome"  : true, /* scroll to top of pad */
     "pageUp"    : true,
-    "pageDown" : true,
+    "pageDown"  : true
   },
 
   /* Should we suppress errors from being visible in the default Pad Text? */