Documentation improvements in object_tracing.c

I've updated docs in object_tracing.c a bit:

* `trace_object_allocations_stop` has improved wording.
* `trace_object_allocations_clear` now has a comment about the consequences on garbage-collecting.

I think that the example in the rdoc for `trace_object_allocations` should feature `allocation_class_path` and `allocation_method_id` but Koichi-san might want to do this in a specific way.