Add some more documentation for \defaultchild (issue 8474044)

[tdanielsmusic]

Minor changes to phrasing suggested.
I don't think this needs an example; seems pretty clear to me.  So LGTM.



https://codereview.appspot.com/8474044/diff/1/Documentation/notation/changing-defaults.itely
File Documentation/notation/changing-defaults.itely (right):

https://codereview.appspot.com/8474044/diff/1/Documentation/notation/changing-defaults.itely#newcode1328
Documentation/notation/changing-defaults.itely:1328: context is created
until getting to a context without a
... an event is encountered, further subcontexts are created
recursively, each of type given by the respective value of
@samp{defaultchild}, until a context with no @samp{defaultchild} setting
is reached.

https://codereview.appspot.com/8474044/diff/1/Documentation/notation/changing-defaults.itely#newcode1332
Documentation/notation/changing-defaults.itely:1332: staves or scores.
Using @code{\new} for explicitly creating
... @code{new} to create contexts explicitly avoids ...

https://codereview.appspot.com/8474044/