Doc help: \markup{} vs \mark

[Graham Percival]

What's the actual difference between \markup and \mark?

Historically, \mark was used for rehearsal marks, and the docs reflect
that.  These days, however, \mark is used for things like fermatas on
bar lines, "DS. al Fine" right-aligned at the end of a piece, etc.

Is the "definition" of a \mark simply that it is a grob that's placed 
shortly
before the next note, on a barline if possible ?  experimentation
suggests that; is there anything more to know about \mark ?

\relative c'' {
c4 \mark \default
c2 \mark \default
c4 \mark \default
c1 \mark \default
}

(Mats: I sent this to -devel instead of -user, so I'd appreciate your 
help.  :)

Cheers,
- Graham