make cut a command, and add documentation

lib/ace/commands/default_commands.js

@@ -293,6 +293,18 @@ exports.commands = [{
 
 // commands disabled in readOnly mode
 {
+    name: "cut",
+    exec: function(editor) {
+        var range = editor.getSelectionRange();
+        editor._emit("cut", range);
+
+        if (!editor.selection.isEmpty()) {
+            editor.session.remove(range);
+            editor.clearSelection();
+        }
+    },
+    multiSelectAction: "forEach"
+}, {
     name: "removeline",
     bindKey: bindKey("Ctrl-D", "Command-D"),
     exec: function(editor) { editor.removeLines(); },