From FAQ:
How can I share my screen?
During a call, right-click on the correspondence screen.
Select the “Screen Sharing” option.
Your screen is now shared.
Hi @msuchanek, which client are you using? It works for me on the GNOME client for Linux. Just right click during a video call (on the video of the other person). I don’t know if it works yet on Jami for Windows.
Jami is going through big changes right now, so not everything on the website is completely up-to-date. If you think of improvements to the software or the documentation you can also open issues on https://git.jami.net 
So again - what is ‘correspondence screen’? As said, there is absolutely no documentation so all I could do was randomly clicking some elements in the UI, and none of them provides an option to share desktop.
And again, why do I have to right-click some element? Why can’t there be an easy to discover UI element such as a button?
What is GNOME client? I do not see any offered, and I would not use it as I do not run GNOME. I tried the .deb and .rpm packages and neither offers the option as far as I can tell.
‘correspondence screen’ is the video of the other person.
You are running the GNOME client on Linux (it’s really a GTK client). There is also a QT client but it’s not available as a package yet.
It looks like your issue is a duplicate of https://git.jami.net/savoirfairelinux/ring-project/issues/737 so the developers know about it and will work on it when they can. Thanks for bringing the FAQ wording to our attention 
Remember that most of the developers are from Quebec so they speak French natively and the English translations are not always 100% perfect.