doc/style: fix doxygen error

doc/style: fix doxygen error

Doxygen complains about non-closed nested comments:
	doc/manual/style.txt:423: warning: Reached end of file
	while still inside a (nested) comment. Nesting level 1
	(probable line reference: 149)

This is caused by the string '/**' that is interpreted as the
beginning of a comment.

Escape the string to not let doxygen consider it as a comment

While there, replace @code/@endcode with @verbatim/@endverbatim to
properly render the line.

Change-Id: If2a27c4cf659326e317cc4ac8c0b313e97e40432
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5996
Tested-by: jenkins