Doc addition: TextSpanner vs. crescendoSpanner

[John Mandereau]

Hi Valentin,

I'm not the only guy who tries to make a crescendo spanner with a
TextSpanner, whereas setting crescendoSpanner (and optionally
crescendoText) is often more suitable, as shown in
http://lists.gnu.org/archive/html/lilypond-user/2008-03/msg00352.html

What about adding a sentence about this, and a reference to Dynamics in
'See also' section in 'Text spanners'?  As Dynamics is quite a long
node, it is probably a good idea to add
changing-text-and-spanner-styles-for-text-dynamics.ly in Selected
snippets too.

Hope this helps,
John