Re: NR: explain that \type shouldn't be used in derived and midi contexts (issue 13578045)

[dak]

https://codereview.appspot.com/13578045/diff/4001/Documentation/notation/changing-defaults.itely
File Documentation/notation/changing-defaults.itely (right):

https://codereview.appspot.com/13578045/diff/4001/Documentation/notation/changing-defaults.itely#newcode1194
Documentation/notation/changing-defaults.itely:1194: context.
That puts out conflicting information since some lines above it is
stated
"This should always be @code{Engraver_group}."

The section needs to make clear that it is written with the example of a
\layout definition in particular, establishing that there are multiple
kinds of output definition, mentioning when something is only specific
to \layout, and make clear that one needs to extend output definitions
for all kinds of output one actually wants to generate.

But in this form, conflicting information is given in relation to a use
case that is not the topic of this section, without putting it in
context well enough that people actually know what and why they are
supposed to be doing.

https://codereview.appspot.com/13578045/