Part-combiner: Add documentation about overriding functions (issue4132045)

[percival . music . ca]

http://codereview.appspot.com/4132045/diff/1/Documentation/notation/simultaneous.itely
File Documentation/notation/simultaneous.itely (right):

http://codereview.appspot.com/4132045/diff/1/Documentation/notation/simultaneous.itely#newcode842
Documentation/notation/simultaneous.itely:842: if one or two notes
actually concide and could be printed as unisono. For this
"coincide"

http://codereview.appspot.com/4132045/diff/1/Documentation/notation/simultaneous.itely#newcode846
Documentation/notation/simultaneous.itely:846: back to the automatically
determined strategy, use @code{\partcombineAutomatic}.
I suggesting removing this sentence and just explaining it in the list.

http://codereview.appspot.com/4132045/diff/1/Documentation/notation/simultaneous.itely#newcode848
Documentation/notation/simultaneous.itely:848: @itemize @bullet
doc policy is not to include the @bullet.

http://codereview.appspot.com/4132045/diff/1/Documentation/notation/simultaneous.itely#newcode887
Documentation/notation/simultaneous.itely:887: e e |
technically all of these should have a 2 duration (we want explicit
durations for the first note of every line)

http://codereview.appspot.com/4132045/