docs addition, beaming cadenzas

[Keith E OHara]

Dear documentation editors,
  In the existing @knownissues for cadenzaOn, I suggest adding one sentence (in 
context below) encouraging manual beams. Users might *think* autobeaming works 
through cadenzas, and it does for a while, but it will not through longer 
cadenzas.

  There is a related low-priority issue 1289, but the suggested text remains 
true and helpful after any of the proposed resolutions to that issue.

  The immediately-following example depended on autobeaming, I made its beam 
manual. (While I'm at it, I suggest this one be an @example, because in this 
case the _code_ is the point, and the image gives no useful information.)

  Two earlier examples use autobeaming in their cadenzas, so I marked the two 
affected lines below, but this detail is not worth much effort.

Thanks,
   Keith



Notation Reference, Rhythms, 1.2.3, Unmetered Music

1348 c4 c d8[ d d] f4 g4.

1362 c4 c d8[ d d] f4 g4.


1402 @knownissues
1403
1404 LilyPond will insert line breaks and page breaks only at a
1405 bar line.  Unless the unmetered music ends before the end of the
1406 staff line, you will need to insert invisible bar lines with
1407
1408 @example
1409 \bar ""
1410 @end example
1411
1412 @noindent
1413 to indicate where breaks can occur.
+ Automatic beaming rules are unlikely to produce the desired beams
+ in cadenzas longer than one or two beats, so it is best to specify beams
+ manually with @code{[} and @code{]}.
+
+ If your piece begins with @code{\cadenzaOn}, then be sure to explicitly
+ create a @code{Voice} context, otherwise strange errors may occur.
+
+ @example
+  \new Voice \relative c'{
+   \cadenzaOn
+   c16[^"Solo Free Time" d e f] g2. \bar "||"
+   \cadenzaOff
+ }
+ @end example