code: @Nonnull
    public DigestAuthServerBuilder setRealm (@Nonnull final String sRealm)
    {
      if (!HttpStringHelper.isQuotedTextContent (sRealm))
        throw new IllegalArgumentException ("realm is invalid: " + sRealm);
  
      m_sRealm = sRealm;
      return this;
    }
docs: A string to be displayed to users so they know which username and password
  to use. This string should contain at least the name of the host performing
  the authentication and might additionally indicate the collection of users
  who might have access. An example might be
  "registered_users@gotham.news.com".
  
  @param sRealm
  The realm to be used. May not be <code>null</code> and should not be
  empty.
  @return this