Add documentation for text dynamic spanners like \cresc

[Reinhold Kainhofer]

Our documentation currently still advocates \crescTextCresc c2\< for creating 
text spanners for crescendi:
http://kainhofer.com/~lilypond/Documentation/notation/expressive-marks-
attached-to-notes.html#dynamics

Since in 2.13.x we now have the nice \cresc text crescendo functions, the 
documentation should be adjusted to document and recomment those text dynamic 
spanners instead (which do not require one command before the note and the \< 
after the note).

Cheers,
Reinhold
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Reinhold Kainhofer, address@hidden, http://reinhold.kainhofer.com/
 * Financial & Actuarial Math., Vienna Univ. of Technology, Austria
 * http://www.fam.tuwien.ac.at/, DVR: 0005886
 * LilyPond, Music typesetting, http://www.lilypond.org