API doc: use minus sign (U+2212) where appropriate instead of `-`

API doc: use minus sign (U+2212) where appropriate instead of `-`

The minus sign is usually rendered larger than `-` and therefore
visually more interesting for mathematical expressions.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I9f9dc8cde4ecce1b40e9fb08ac2e188221fffe3b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4527
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>