ftrace: Fix most kernel-doc warnings

Reduce the number of kernel-doc warnings from 52 down to 10, i.e.,
fix 42 kernel-doc warnings by (a) using the Returns: format for
function return values or (b) using "@var:" instead of "@var -"
for function parameter descriptions.

Fix one return values list so that it is formatted correctly when
rendered for output.

Spell "non-zero" with a hyphen in several places.

Link: https://lore.kernel.org/linux-trace-kernel/20240223054833.15471-1-rdunlap@infradead.org

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/oe-kbuild-all/202312180518.X6fRyDSN-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>