While the 'ansible-playbook' command will look for {host,group}_vars in
both the playbook and inventory directories, the 'ansible' command
will only look in the inventory directory. This follows from a close
reading of the documentation but isn't explicit and leads to the
perhaps unexpected result that you can run both commands with the same
configuration in the same directory and get different results.

This commit adds a note to the documentation making the difference in
behavior explicit.

Closes ansible#48065