removing \markup "foo" from the NR

[Graham Percival]

Should we discuss
  \markup " foo "
?

IMO, this only leads to confusion:
- markup \commands inside "" are interpreted literally.
- you cannot nest expressions inside "".
- lilypond already uses a lot of {} expressions, so limiting
  newbies to only \markup {} isn't such a bad thing.

I propose that we only mention
  \markup "foo"
as well as
  \markup foo
in the @knownissues.  This strikes me as something similar to the
"\relative {" issue.

Cheers,
- Graham