Document reusing parameters in run_command_on_placements/shards

One of the most common questions in public slack is how to use the table name in `run_command_on_placements` more than once.

The template is passed through [format](https://www.postgresql.org/docs/current/functions-string.html#FUNCTIONS-STRING-FORMAT), which allows you to use a parameter multiple times by using `%1$s` instead of `%s`, or `%1$L` is you want it to be quoted as a string.