Doc: Clarify grace note synchronisation workaround (issue 90270044)

[lilyliska]

Reviewers: ,

Description:
Doc: Clarify grace note synchronisation workaround

As pointed out in the thread
http://lists.gnu.org/archive/html/lilypond-user/2014-04/msg00540.html
it is necessary to use \grace for the old grace note sync issue.
This commit makes that explicit in the known issues section in the NR

Please review this at https://codereview.appspot.com/90270044/

Affected files (+6, -0 lines):
  M Documentation/notation/rhythms.itely


Index: Documentation/notation/rhythms.itely
diff --git a/Documentation/notation/rhythms.itely  
b/Documentation/notation/rhythms.itely
index  
9ca613ed753d4e025dac2bbfdcd9d3a4fb3ce5b1..51178310127983208de660bc91adee23d4a53e4e  
100644
--- a/Documentation/notation/rhythms.itely
+++ b/Documentation/notation/rhythms.itely
@@ -3511,6 +3511,12 @@ durations in the other staves.  For the above example
 >>
 @end lilypond

+Please make sure that you use the @cmd{\grace} command for the
+spacer part, even if the visual part uses @cmd{\acciaccatura} or
address@hidden because otherwise an ugly slur fragment will
+be printed, connecting the invisible grace note with the following
+note.
+
 The use of grace notes within voice contexts confuses the way the voice
 is typeset. This can be overcome by inserting a rest or note between the
 voice command and the grace note.