Skips entries to the first beyond the current whose document number is
/// greater than or equal to target. Returns true iff there is such
/// an entry. Behaves as if written:
/// boolean skipTo(int target) {
/// do {
/// if (!next())
/// return false;
/// } while (target > doc());
/// return true;
/// }
///
/// Some implementations are considerably more efficient than that.
///
bool SkipTo(int target);
///