[SYCL][Doc] Relax profiling tag timestamp reqs (#22847)
We discovered that setting the `command_submit` timestamp is adding
excessive overhead to `enqueue_signal_event` and `submit_profiling_tag`.
These APIs are normally used only to get the `command_end` timestamp,
though, so paying extra overhead to set `command_submit` did not make
sense. Relax the guarantees for `command_submit`, so that
implementations can simply set it to the same value as `command_end`.
Also clarify that `command_start` is the same as `command_end` for these
APIs. Because they submit an empty command, the hypothetical command's
"start" and "end" happen at the same time.
