[CLI help] add-cloud and add-credential are inconsistent

The help for `add-cloud` shows a usage of:

juju add-cloud [options] <cloud name> <cloud definition file>

but it also lists the -f option for specifying the definition file.

The provided example does not use the -f option.

This command supports both forms.

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The help for `add-credential` shows a usage of:

juju add-credential [options] <cloud name>

and also lists the -f option.

The provided example does use the -f option.

This command only supports the -f form.

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So `add-cloud` is unclear because it shows the -f option that is not required and `add-credential` is wrong in terms of its listed usage. Both commands' help is also inconsistent in terms of supporting the omission of the -f option and the usage of the -f option in their examples.

I'm not sure if other commands can be used for comparison purposes in order to arrive at a consistent software design and CLI help.