Added vowel transitions for lyrics

Documentation/notation/vocal.itely

 @c -*- coding: utf-8; mode: texinfo; -*-
 @ignore
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 @c \version "2.21.0"
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 @seealso
 Internals Reference:
 @rinternals{LyricExtender},
 @rinternals{LyricHyphen}.
 
 
 @node Gradual changes of vowel
 @unnumberedsubsubsec Gradual changes of vowel
 
-@cindex transition
-
-Gradual changes of vowel may be indicated with transition arrows,
-which are entered as @samp{->} between syllables (see Gould pp.
-452--453).  The arrow shows the length of the transition.  Transition
-arrows are by default always drawn, with space added if necessary in
-tightly engraved music.  Ties between notes of unchanged pitch or
-slurs show that there is no new articulation, despite a change of
-vowel.  The minimum length of transition arrows may be adjusted with
-the @code{minimum-length} property of @code{LyricTransition}.
+@cindex vowel transition
+
+Vowel transitions (gradual changes of vowel or sustained
+consonant), which may be indicated by arrows between syllables,
+are entered with the command @code{\vowelTransition} (see
+Gould pp. 452--453).  The arrow shows the length of
+the transition, and it is by default always drawn (space is
+added if necessary in tightly engraved music).  Ties between
+notes of unchanged pitch or slurs show that there is no
+new articulation, despite a change of vowel.  The minimum length
+of the arrows may be adjusted with the @code{minimum-length}
+property of @code{VowelTransition}.
 
 @lilypond[verbatim,quote]
 {
   c'2 c'
   \set melismaBusyProperties = #'()
   c'2 ~ c'
   \time 3/2
   d'4( e' f' e' f'2
   e'2.)
 }
 \addlyrics
 {
-  Ah -> oh,
-  ah -> oh,
-  ah -> _ _ _ _
+  Ah \vowelTransition oh,
+  ah \vowelTransition oh,
+  ah \vowelTransition _ _ _ _
   oh.
 }
 @end lilypond
 
 @seealso
 Musical Glossary:
-@rglos{transition arrow}.
+@rglos{vowel transition}.
 
 Internals Reference:
-@rinternals{LyricTransition}.
+@rinternals{VowelTransition}.
 
 
 @node Techniques specific to lyrics
 @subsection Techniques specific to lyrics
 
 @c TODO this whole section is to be reorganised
 
 @menu
 * Working with lyrics and variables::
 * Placing lyrics vertically::
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 @c Add "Printing both the ancient and the modern clef in vocal music" snippet,
 @c and "Transcription of Ancient music with incipit" snippet. -vv
 
 @seealso
 Notation Reference:
 @ref{Ancient notation}.