Doc: mention empty chords; avoid using zero-duration spacers in examples

Documentation/notation/keyboards.itely

 @c -*- coding: utf-8; mode: texinfo; -*-
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 @c \version "2.14.0"
@@ skipping 189 matching lines @@
 @cindex collisions, cross-staff voices
 
 Overlapping notation can result when voices cross staves:
 
 @lilypond[verbatim,quote]
 \new PianoStaff <<
   \new Staff = "up" {
     \voiceOne
     % Make space for fingering in the cross-staff voice
     \once\override DynamicLineSpanner #'staff-padding = #3.4
-    e''2\p\< d''\> | c''1\!

[dak, 2012/05/13 19:06:43]

I am not particularly happy about "defusing" the demonstrated problem with
additional material.

[Keith, 2012/05/14 06:55:08]

This example is rather dense.  Any other way of defusing the s1*0 problem is
distracting, as was the s1*0 itself.

+    e''2\p\< d''\>
+    c''1\!
   }
   \new Staff = "down" <<
     {
       \clef bass
-      s4. e,8\rest g,2\rest | c1
+      s4. e,8\rest g,2\rest
+      c1
     } \\ {
       c8\( g c'
       \change Staff = "up"
-      e' g' b'-3 a' g'\) | f'1
+      e' g' b'-3 a' g'\)
+      f'1
     }
   >>
 >>
 @end lilypond
 
 The stem and slur overlap the intervening line of dynamics
 because automatic collision resolution is suspended for beams, slurs
 and other spanners that connect notes on different staves,
 as well as for stems and articulations if their placement is
 affected by a cross-staff spanner.
@@ skipping 446 matching lines @@
 The @code{\harp-pedal} command accepts a string of characters, where
 @code{^} is the highest pedal position (flattened pitch), @code{-} is
 the middle pedal position (natural pitch), @code{v} is the lowest pedal
 position (sharpened pitch), and @code{|} is the divider.  A prefixed
 @code{o} will circle the following pedal symbol.
 
 @seealso
 Notation Reference:
 @ref{Text scripts},
 @ref{Instrument Specific Markup}.