Hi Helena and Beatriz,
It is possible to edit the section summary just like Beatriz already mentioned. There is however a caution to this: if you (by accident) happen to delete the section, you'll lose the text you added without any way to restore it.
If you add the text using the Label resource, this has some important advantages:
the text can be moved to another section, or right between two
activities in the section
when you delete the section, the text goes to the recycle bin, being a Label resource that may be restored later on.