usage example for pure-unpure container sought

[Werner LEMBERG]

I've found some minor documentation and formatting issues in NR
section 5.5.6, "Unpure-pure containers", which I'm going to fix.  In
general, however, there is a serious problem with the example in this
subsubsection, as documented in issue #3512:

  https://gitlab.com/lilypond/lilypond/-/issues/3512

The given example no longer works, or rather, the proposed solution to
the formatting problem is useless because LilyPond already does the
right thing automatically.

Looking at the current output of regtest `unpure-pure-container.ly` I
have the feeling that the description text doesn't fit the code at
all: It always talks about the second note of each group, while the
code modifies the first note.  Of course, it is also possible that I'm
too dumb to really understand what's going on ...

I thus search a good example for using an unpure-pure container that I
could add to the documentation.  And if someone explains to me how the
regtest works I'm going to reformulate its documentation text, too, so
that even I can understand it :-)


    Werner