Explicitly indicate `--logdir_spec`/`--load_fast` mismatch

[wchargin]

You can’t use the (deprecated) --logdir_spec flag with RustBoard. As
of #4794, we’ll gracefully fall back to the multiplexer if you attempt
to do so. But maybe this particular invocation is common enough that it
merits an explicit explanation of the problem rather than a silent
fallback:

$ tensorboard --logdir_spec foobar --load_fast auto
warning: --logdir_spec not supported with --load_fast; falling back to
slow data loading paths
TensorBoard 2.5.0a0 at ...