Doc: Add a section about handling MIDI dynamics with multiple voices (issue 302930043 by address@hidden)

[ht . lilypond . development]

Reviewers: ,

Description:
Doc: Add a section about handling MIDI dynamics with multiple voices

As MIDI dynamics of every Voice are by default independent of
dynamic changes occurring in all other Voices, writing LilyPond
input using multiple voices may require some additional
considerations to be taken into account to keep in full control
of MIDI volume for MIDI output.

(While users interested only in typeset output can safely - and
probably do - ignore these considerations, users who also wish to
generate MIDI output can easily remain unaware of the issues
with MIDI dynamics that ignoring them may cause, because the need
for such considerations is not raised directly anywhere in the
documentation.  See, for example, the full disussion thread
<http://lists.gnu.org/archive/html/lilypond-user/2014-05/msg00361.html>
for comments from both groups of users, and the false assumptions
that could be made about MIDI dynamics in output generated from
multiple voices when relating it to the typeset output.)

This change in the documentation tries to describe in more detail
how MIDI dynamics interact with voices, and, now that the last
pieces of source code logic (which had become less and less
effective over time, and had not been complete even when they
were first introduced) of signaling warnings about cases
where the MIDI output might not fully match the expected one
have been removed as a result of the code changes related to
Issue 3945, lists some simple guidelines to help users keep in
control of MIDI volume with multiple voices.

The change consists of a forward reference to a new MIDI
subsection in the beginning of the section about single-staff
polyphonic notation (where the syntax of defining Voices is
described), and a new MIDI subsection which illustrates the
default interaction of MIDI dynamics and voices by way of an
example.

Please review this at https://codereview.appspot.com/302930043/

Affected files (+121, -1 lines):
  M Documentation/notation/input.itely
  M Documentation/notation/simultaneous.itely