Add detailed documentation for scheduler "tuning" parameters

[ashb]

#12139 added some new params to the scheduler -- we have "cursory" docs for them in the comments in the config file, but we should have a more details docs in the scheduler.rst page.

[mik-laj]

If possible, I would change the option name from clean_tis_without_dagrun to clean_tis_without_dagrun_interval The other configuration options have this suffix.

[potiuk]

Hey @ashb @mik-laj - something for 2.0.0rc1? or will you change the milestone?