docs: Clarify DC_EVENT_INCOMING_WEBXDC_NOTIFY documentation

[Hocuri]

I found the old documentation rather hard to understand. The new doc string:

• uses whole sentences, leaving less space for misinterpretation
• explicitly mentions that it can happen that there is no webxdc-info-message
• is clearly structured using bullet points.

I didn't understand what "in case of summary change" means, maybe this needs to be added to the new doc string.

[r10s]

nice!

the new "Sometimes no webxdc-info-message is added to the chat" explains the "summary change" much better (the situation happens when an update changes update.summary but not update.info at the same time)