Clarify that poetry update only updates main dependencies

[hugobuddel]

Add a note that --with is required to let poetry update also update optional dependency groups.

This behavior of poetry update confused me, so I thought let's try to save other people the trouble by updating the documentation.

I do want to note that I find the behavior strange. If I do poetry install --with=dev followed by poetry update, then I expect all installed dependencies to be updated, also the dev ones.

Pull Request Check List

Resolves: #issue-number-here

• [ ] Added tests for changed code.
• [x] Updated documentation for changed code.