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);