firewire: core: update documentation of kernel APIs for flushing completions

There is a slight difference between fw_iso_context_flush_completions() and
fw_iso_context_schedule_flush_completions().

This commit updates the documentations for them.

Link: https://lore.kernel.org/r/20240912133038.238786-5-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>