IR: Improve performer documentation (issue 6868047)

[dak]

Improving the docs certainly is quite worthwhile.


https://codereview.appspot.com/6868047/diff/1/scm/document-translation.scm
File scm/document-translation.scm (right):

https://codereview.appspot.com/6868047/diff/1/scm/document-translation.scm#newcode21
scm/document-translation.scm:21: ;; type predicates ly:engraver? and
ly:performer?.
I think that this approach is not really helping as it is inflexible and
it becomes harder to see "crossovers".  At one point of time, we will
likely get translators that are used to output MusicXML, and several
translators can be used as either translator or performer (and some,
like the autobeamer, will also be used in MusicXML preparation).  So I'd
rather make separate chapters for translators used in $defaultlayout
(default engravers) and those used in $defaultmidi (default performers),
allow for duplicates but mention when the engraver is also being used as
performer and vice versa, and make a separate list of translators not
used by default (are there any?).

https://codereview.appspot.com/6868047/