MFC 358954:

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