#3315 doc: add Harm's explanation about overriding stencils

[Anonymous]

Originally created by: *anonymous

Originally created by: janek.li...@gmail.com

Harm wrote a very helpful explanation about overriding grob stencils.  It should be added to Extending manual (unless he decides to turn it into a standalone tutorial):

http://lists.gnu.org/archive/html/lilypond-user/2013-04/msg00375.html