Add some more examples about what flattens and what does not

Author: moritz

lib/containers.pod

@@ -293,4 +293,25 @@ scalar container) of the invocant, whereas C<.flat> walks the elements of the
 invocant, and removes their itemization (note that both methods don't modify
 the invocant object; they just return a modified copy).
 
+=item What flattens, what doesn't?
+
+In general, operations that have to iterate the list usually flatten the list.
+Other operations don't. Here is a list of a few operations that flattens:
+
+=item .map, .grep, .first
+=item Iteration with "for"
+=item .join
+=item assignment to an array variable
+=item assignment to a hash variable
+
+And a few that don't flatten:
+
+=item Indexing with [$idx]
+=item .elems
+=item .tree
+
+The first two cannot flatten, otherwise it would be very hard to get access
+to the structure that's actually stored in the list or parcel, and the whole
+point of C<.tree> is not flatten.
+
 =end pod