docs: describe purpose of `odc_admin`, `odc_manage` and `odc_user`

[pjonsson]

After running datacube system init on a fresh database, I had to look in the code to figure out what the created users are called, and according to my git grep they are not mentioned in any file under docs/.

I can guess that odc_admin is meant to have all (useful) permissions, and I realize that odc_manage is meant to manage something, but what is it meant to manage, is it effectively the database admin with slightly reduced permissions or is it odc_manage that is meant to add datasets to the database?