Let private members hide public members of parent class.

For example:

    /** @class Parent */
      /** @method foo */

    /** @class Child @extends Parent */
      /** @method foo @private */

Previously this resulted in Child class docs having method foo from
Parent, as if the private foo in Child didn't exist at all.  This
behavior was quite far from obvious.

Now the Child class docs will have no foo method at all - the parent
class method gets shadowed with @private foo in subclass.  This should
be more like what the documentation author intended.

As a bonus we now have better error messages for cases where {@link}
refers to a private member, and @inheritdoc can be used to inherit
docs from private members.