Add documentation of the URI format in the `--help` output of
`redis-cli` and `redis-benchmark`.

In particular, it's good for users to know that they need to specify
"default" as the username when authenticating without a username. Other
details of the URI format are described too, like scheme and dbnum.

It used to be possible to connect to Redis using an URL with an empty
username, like `redis-cli -u redis://:PASSWORD@localhost:6379/0`. This
was broken in 6.2 (#8048), and there was a discussion about it #9186.
Now, users need to specify "default" as the username and it's better to
document it.

Refer to #12746 for more details.