Document incipit command (issue 232180043 by address@hidden)

[PhilEHolmes]

Reviewers: dak, Trevor Daniels, J_lowe,

Message:
Please review

Description:
Docstring and index entries added

Please review this at https://codereview.appspot.com/232180043/

Affected files (+4, -0 lines):
  M Documentation/notation/ancient.itely
  M ly/property-init.ly


Index: Documentation/notation/ancient.itely
diff --git a/Documentation/notation/ancient.itely  
b/Documentation/notation/ancient.itely
index  
8df2029c62e000c90de37cec14bdeb6cf7840c35..71a2685ff337b2a1f76e5f75692cf962c7c24574  
100644
--- a/Documentation/notation/ancient.itely
+++ b/Documentation/notation/ancient.itely
@@ -2647,6 +2647,8 @@ same source.

 @node Incipits
 @unnumberedsubsubsec Incipits
address@hidden \incipit
address@hidden incipit

 It is customary when transcribing mensural music into modern notation to  
place
 an indication of how the initial rests and note or notes of the original  
version
Index: ly/property-init.ly
diff --git a/ly/property-init.ly b/ly/property-init.ly
index  
8077aa231e3880bcd458d000a2db762702073b35..a50cf82635b857aad7f4327fc65fb5b7516c583c  
100644
--- a/ly/property-init.ly
+++ b/ly/property-init.ly
@@ -278,6 +278,8 @@ improvisationOff = {

 incipit =
 #(define-music-function (parser location incipit-music) (ly:music?)
+  (_i "Output @var{incipit-music} before the main staff as an indication of
+    its appearance in the original music.")
   #{
     \once \override Staff.InstrumentName.stencil =
       #(lambda (grob)