Improve docs for modifying metadata

[benjeffery]

From #3140

"Still, it'd be good to have not just "see here" but maybe a warning that in-place modification wont' work. (Also, those docs don't have the simplest use cases of "change an existing value in top-level metadata" or "change/add a value in each row".)"

[hyanwong]

Also it wasn't clear to me at first sight how to delete top-level metadata (there is no drop_metadata() method at the top level). But it looks like you can just assign e.g. tables.metadata={}.