MFC 358954:
CMSG_DATA.3: Fix formatting of printf escape sequences
Escape sequences like "\n" have to be escaped twice in examples in our
mdoc(7)-based manual pages in order to be displayed properly. The problem
is that otherwise they are interpreted by mdoc(7), which results in:
printf("parent: received '%s'0, buf);
being shown to the user instead of:
printf("parent: received '%s'\n", buf);
Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D24056