I have several .screen files inside /xxx/documentation
and its subdirectories that are already tracked by Git.
After modifying many of these screen files, I
It's a bug in the documentation. Quote the asterisk with
$ git add documentation/\*.screen
or
$ git add 'documentation/*.screen'
to get the behavior you want.
If instead you want to add files in the current directory only, use
$ git add *.screen
UPDATE: I submitted a patch that corrects the issue, now fixed as of version 1.6.6.2.