Docs: Explain the difference between ritardando and rallentando (issue 5544075)

[plroskin]

Reviewers: ,

Message:
The description for rallentando only said that it's an Italian word.
Provide better descriptions for ritardando and rallentando, contrast
them to each other and add references in both directions.

Description:
Docs: Explain the difference between ritardando and rallentando

Please review this at http://codereview.appspot.com/5544075/

Affected files:
  M Documentation/music-glossary.tely


Index: Documentation/music-glossary.tely
diff --git a/Documentation/music-glossary.tely  
b/Documentation/music-glossary.tely
index  
5f113dda908f83874e7e4ff89ba346948dde190d..b2df038dc722d5ec91047f054db0d7c9e0ccfe1f  
100644
--- a/Documentation/music-glossary.tely
+++ b/Documentation/music-glossary.tely
@@ -6644,7 +6644,10 @@ DK: rallentando,
 S: rallentando,
 FI: rallerdando, hidastuen.

-[Italian] A performance indication, abbreviated @notation{rall.}
+[Italian: @q{slowing down}]
+
+Slackening in speed, more gradual than @ref{ritardando}.  Abbreviated
+to @notation{rall.}

 @seealso
 @ref{ritardando}.
@@ -6770,11 +6773,13 @@ DK: ritardando,
 S: ritardando,
 FI: ritardando, hidastuen,

-Gradually slackening in speed.  Mostly abbreviated to @notation{rit.} or
address@hidden
+[Italian: @q{lagging}]
+
+Gradual slowing down, more pronounced than @ref{rallentando}.  Mostly
+abbreviated to @notation{rit.} or @notation{ritard}.

 @seealso
-No cross-references.
address@hidden


 @node ritenuto