Clarify the doc between project create vs update.

[ericbl]

Current documentation leads to confusion between "capycli project create" and "capycli project update".
Looking at the code, both are doing the same stuff, but with the major difference being the onlyUpdateProject==True set in https://github.com/sw360/capycli/blob/main/capycli/project/create_project.py.
Looking further, this is setting the 'add' in https://github.com/sw360/sw360python/blob/master/sw360/project.py / update_project_releases(self, releases, project_id, add=False):
which says
Ifaddis True, givenreleases are added to the project, otherwise, the existing releases will be replaced.
And the code indeed create a list of releases to be linked to the project by getting the existing ones and adding the new ones.

Therefore, 'capycli project create' instead of 'capycli project update' would update the project but replace all releases, i.e. remove existing ones.
'create' is actually the only option if we want to update an existing project but remove some releases that do not belong to it anymore.

Please clarify the doc to reveal this major difference.

[gernot-h]

It might be that I once added "project update" as a 2nd way to call "project create" with somewhat "safer" default settings. So the idea was that "project update" will refuse to create a new project and carefully update an existing one without overwriting things.

I admit this might be a bit confusing. So before we improve the documentation, do we want to keep that behaviour? @tngraf, what do you think?