lttng-disable-event(1): `--tracepoint` option is not the default

I previously thought that not specifying any instrumentation point type
option was equivalent to specifying `--tracepoint`, like with the
`enable-event` command.

It's not the case: `--tracepoint` is just a filter, like `--syscall` and
the others. With `--kernel`, if you don't specify any instrumentation
point type condition option, the `disable-event` command disables
recording event rules regardless of their instrumentation point type.

This patch updates lttng-disable-event(1) to explain this.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I489c5d16dae954f4b5116edce556fdd942c98c4d