Doc: improve NR 3.4.2 skipTypesetting subsec.
See https://lists.gnu.org/archive/html/bug-lilypond/2020-06/msg00019.html for context.
/cc @tdanielsmusic
See https://lists.gnu.org/archive/html/bug-lilypond/2020-06/msg00019.html for context.
/cc @tdanielsmusic