I figured it out. [From the documentation][1]: **`(0W8`**. [I found the following somewhat related answer on another SE site][2], 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);


  [1]: http://vimdoc.sourceforge.net/htmldoc/indent.html#cinoptions-values
  [2]: https://stackoverflow.com/a/699313/1022889