Refine API GC setters and getter comments regarding locking.

Refine API GC setters and getter comments regarding locking.

* include/gc_mark.h (GC_set_start_callback): Refine comment (the
function acquires the GC lock).
* include/gc.h (GC_set_all_interior_pointers, GC_set_stop_func,
GC_get_stack_base): Likewise.
* include/gc.h (GC_get_version, GC_get_gc_no, GC_get_parallel,
GC_set_find_leak, GC_get_all_interior_pointers,
GC_get_dont_precollect, GC_get_pages_executable, GC_set_max_heap_size,
GC_set_force_unmap_on_gcollect): Refine comment (the function is
lock-free).