Markup vertical alignment

[Phil Holmes]

Have a look at 
http://lilypond.org/doc/v2.21/Documentation/notation/formatting-text#text-alignment - 
the bit starting "Vertical alignment is a bit more complex".  It says "the 
element to be moved needs to be preceded with an anchor point".   Well - the 
positioning of une foret shows clearly that it doesn't.  Trawling through my 
collection of Lilypond executable, it look like it changed between 2.17.26 
and .28.

I've not grasped the current process well enough to do a patch for review, 
but this looks trivial enough for a simple push/merge request.  Could 
someone oblige?

--
Phil Holmes