lttng: enable-event: use the terminology of the documentation

lttng: enable-event: use the terminology of the documentation

Rework most of the human-readable messages of the enable-event to use
the terminology used throughout the online documentation and the man
pages.

Some clean-ups are also done to follow the rest of the project's
conventions, such as quoting user input with back-ticks, not ending
messages with a period, etc.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I00d89d6e3c32ccbde60081ef427a099fb8cd206e