Update backfills docs with more up to date info on single runs

[Rhiyo]

Summary & Motivation

I noticed the docs of the backfill page for single runs are out of date with 1.8 based on the changes here: https://github.com/dagster-io/dagster/pull/21259

Starting this PR to correct that - the note on the backfill was incorrent in its current form.

• potentially should add an example when using with jobs
• add an example with multipartitions (Static + Date Range)

It seems if you do try and run a job with multiple different backfill polices it will default to the policy with least max_partitions_per_run, failing that it will default to using multi_run(max_partitions_per_run=1)