HID: retrode: tell what a Retrode is and drop a blank line

Add descriptive info to prompt string so that someone can know what
a Retrode is.  Drop an unneeded blank line.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Bastien Nocera <hadess@hadess.net>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>