Doc: LM 4.1.2 additonal para for positioning grobs

Issue 4137
As reported in

http://lists.gnu.org/archive/html/bug-lilypond/2014-09/msg00025.html

Added paragraph regarding grobs that don't print output
but are used for 'positioning' and 'managing' other
grobs

diff --git a/Documentation/learning/tweaks.itely b/Documentation/learning/tweaks.itely
index ac1be581b04b56b89c562f7c02af26fe70d4529f..160c0e390114e773964043530e9f7d29e1699d87 100644 (file)
--- a/Documentation/learning/tweaks.itely
+++ b/Documentation/learning/tweaks.itely
@@ -107,6 +107,14 @@ ending point, and maybe other properties concerned with their
 shape.  Objects with an extended shape like these are called
 @q{Spanners}.
 
+What is more, there are @q{abstract} grobs which do not print
+anything of their own, but rather collect, position and manage
+other grobs.  Common examples for this are
+@code{DynamicLineSpanner}, @code{BreakAlignment},
+@code{NoteColumn}, @code{VerticalAxisGroup},
+@code{NonMusicalPaperColumn} and similar.  We will see how some of
+these are used later.
+
 It remains to explain what @q{Interfaces} are.  Many objects, even
 though they are quite different, share common features which need to
 be processed in the same way.  For example, all grobs have a color, a