Added documentation and small typing fix

src/lib/select/select.ts

@@ -152,7 +152,7 @@ export class MdSelect implements AfterContentInit, OnDestroy, OnInit, ControlVal
   private _multiple: boolean = false;
 
   /** Comparison function to specify which option is displayed. Defaults to object equality. */
-  private _compareWith: ((o1: any, o2: any) => boolean) | null = null;
+  private _compareWith: (o1: any, o2: any) => boolean;
 
   /** Deals with the selection logic. */
   _selectionModel: SelectionModel<MdOption>;
@@ -266,6 +266,11 @@ export class MdSelect implements AfterContentInit, OnDestroy, OnInit, ControlVal
     this._multiple = coerceBooleanProperty(value);
   }
 
+  /**
+   * A function to compare the option values with the selected values. The first argument
+   * is a value from an option. The second is a value from the selection. A boolean
+   * should be returned.
+   */
   @Input()
   get compareWith() { return this._compareWith; }
   set compareWith(fn: (o1: any, o2: any) => boolean) {