Doc: NR 5.5.5 Adv tweaks - Unpure-pure containers (issue 5569050)

[mtsolo]

Looks good!  Much more user-understandable than anything I could have
ever come up with :)


http://codereview.appspot.com/5569050/diff/1/Documentation/notation/changing-defaults.itely
File Documentation/notation/changing-defaults.itely (right):

http://codereview.appspot.com/5569050/diff/1/Documentation/notation/changing-defaults.itely#newcode3714
Documentation/notation/changing-defaults.itely:3714: evaluated
correctly, usually returning a value of @var{0} or
It's not that the function is not evaluated correctly - it is not
evaluated at all, and lilypond subs in a dummy value.

http://codereview.appspot.com/5569050/diff/1/Documentation/notation/changing-defaults.itely#newcode3717
Documentation/notation/changing-defaults.itely:3717:
The definition of a pure function is that it does not result in
properties or objects being set or grob suicides.

http://codereview.appspot.com/5569050/