Add note about multisite usage in `wp db query` docs

[Scotchester]

I was caught out by the fact that the global --url parameter, normally used to target a non-primary site in a multisite setup, has no effect on the wb db query command, so I thought I'd propose adding a note about it.

[ernilambar]

@Scotchester Doc markdown page for the commands are generated automatically from the command docs. So it should not be edited directly. The docs you edited is coming from https://github.com/wp-cli/db-command/blob/main/src/DB_Command.php#L398

[Scotchester]

Apologies! I promise I did read the relevant section in CONTRIBUTING.md, but misunderstood it :) Will take my PR to that repo.