doc: [1/3] uniform the texinfo syntax for commands definition

To avoid errors in the documentation, like the one fixed by change
http://openocd.zylin.com/6134/ , use a uniform notation across the
file so simple copy-paste will work.

Both 'Command' and '{Command}' are in use, with the following
statistics:
	  0 @deffnx {Command}
	 45 @deffn {Command}
	 31 @deffnx Command
	382 @deffn Command

While 'Command' is the most popular, prefer the version within
curly-brackets that has to be used for multi-word definition like
'{NAND Driver}', '{Config Command}', '{FPGA Driver}', ...

Patch generated through:
	sed -i 's/^\(@deffn \)\(Command\)/\1{\2}/' doc/openocd.texi
	sed -i 's/^\(@deffnx \)\(Command\)/\1{\2}/' doc/openocd.texi

Change-Id: If692bbf7e546c5287f466a6aa6940d42b3d4655d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6150
Reviewed-by: Yasushi SHOJI <yashi@spacecubics.com>
Tested-by: jenkins