Advances to the first beyond the current whose document number is greater
/// than or equal to target. Returns the current document number or
/// {@link #NO_MORE_DOCS} if there are no more docs in the set.
///
/// Behaves as if written:
///
///
/// int advance(int target) {
/// int doc;
/// while ((doc = nextDoc()) < target) {
/// }
/// return doc;
/// }
///
///
/// Some implementations are considerably more efficient than that.
///
/// NOTE: certain implemenations may return a different value (each
/// time) if called several times in a row with the same target.
///
/// NOTE: this method may be called with {@value #NO_MORE_DOCS} for
/// efficiency by some Scorers. If your implementation cannot efficiently
/// determine that it should exhaust, it is recommended that you check for that
/// value in each call to this method.
///
/// NOTE: after the iterator has exhausted you should not call this
/// method, as it may result in unpredicted behavior.
///
/// NOTE: in 3.0 this method will become abstract, following the removal
/// of {@link #SkipTo(int)}.
///
///
///