ENH: Set the preferred Doxygen documentation style explictly.

Set the preferred Doxygen documentation style explicityle following the
discussion in:
http://review.source.kitware.com/#/c/23399/

Author: Jon Haitz Legarreta

SoftwareGuide/Latex/Appendices/CodingStyleGuide.tex

@@ -271,6 +271,9 @@ \subsection{Doxygen Documentation System}
 Doxygen requires the embedding of simple comments in the code which is in turn
 extracted and formatted into documentation.
 
+Note that ITK prefers the backslash (``\\'') style versus the at-sign (``@'')
+style to write the documentation commands.
+
 For more information about Doxygen, please visit
 \href{http://www.stack.nl/~dimitri/doxygen/}{http://www.stack.nl/~dimitri/doxygen/}