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I figured it out. From the documentation: (0W8. I found the following somewhat related answer on another SE site, to explain why I use a double-indent for continuation lines (i.e. Google and Oracle style guides encourage it).

WN    When in unclosed parentheses and N is non-zero and either
      using "(0" or "u0", respectively and the unclosed parentheses is
      the last non-white character in its line and it is not the
      closing parentheses, indent the following line N characters
      relative to the outer context (i.e. start of the line or the
      next unclosed parentheses).  (default: 0).

    cino=(0            cino=(0,W4
      a_long_line(          a_long_line(
              argument,     argument,
              argument);    argument);
      a_short_line(argument,    a_short_line(argument,
               argument);        argument);