Improve --help documentation to match full documentation

[sefeg]

In the last user test iteration, we commonly observed analysts attempting to use cap-client --help to find a command to perform a desired task. They drew back on --help, even though the full documentation was already loaded in the browser. While the full documentation proved to be helpful for all tasks, this was not the case for the command overview resulting from --help. For example, a user who wanted to update an analysis field, did not manage doing so with the command line --help. It only lists metdata with the description Metadata managing commands. As we see that the full documentation is effective, improving the --help overview by drawing back on already existing descriptions should improve the usability.