Documentation for ocm download cli missing substantial things

[morri-son]

What happened:
Like described in open-component-model/ocm#1029 the ocm download clidoes not work like expected. Besides the fact that it technically does not work, the documentation for that command seems to be buggy and needs to be improved. An end user has to understand what he may miss in the command and its options when getting an unexpected result. There should be examples of the basic scenarios and also an explanation of the result a download has, which is a complex folder structure, here assuming the option -O /tmp for the target folder has been specified:

/tmp/ocm
└── ocm.software
    └── ocmcli
        ├── 0.1.0-alpha.2
        ├── 0.1.0-dev
        ├── 0.10.0
        ├── 0.11.0
        ├── 0.12.0
        ├── 0.3.0-rc.2
        └── 0.9.0
         ...

What you expected to happen:
The documentation should be more precise and focus on the standard scenario. This scenario should have an example.