Here's some proposed language for the docs:
If three or more notes line up in the same column, merge-differently-headed
can't
successfully complete the merge of the two notes that should be merged. To
allow the
merge to complete, apply a \shift to the note that should not be merged.
% \shiftXxx can be used to avoid "warning: too many clashing note
columns"
% the first shared note, d''2 and d''8 is shared between voice1 and voice2
% the second shared note, e,8 and e'2, is shared between voice2 and voice3
% an error message is given, and the notehead is _not_ a half note
\version "2.10.5" %Windows XP
\paper {indent = 0\mm ragged-right = ##t}
\relative c' { \override Staff.NoteCollision #'merge-differently-headed = ##t
<< {d=''2 g2 } \\ {\stemNeutral d=''8 c8 r4 e,8 c'8 r4 } \\ { \voiceFour e,,2 e'2} >>
<< {d=''2 \shiftOn g2 } \\ {\stemNeutral d=''8 c8 r4 e,8 c'8 r4 } \\ {
\voiceFour
e,,2 e'2} >>
}
% end of .ly snippet.