Documentation improvments

[Zykino]

Description

1. I don't see the new tab_flashing_bell_indicator and tab_bell_indicator in the wiki/doc.
2. I'm not sure if there are other "involuntarily hidden" features
3. It would be nice to have a little comparison between the pipe and notification's widget. Just reading the doc make them looks quite redundant.
4. How does the command widget know from which pane to git branch from? (I will test for myself but if it is in the doc it could help understand at a glance)