add new tor circuit feature to mobile tor docs

1. closes #179 (closed)

2. In reference to, https://blog.torproject.org/new-release-tor-browser-140/

What's new? New circuit for Android

In previous versions of Tor Browser for Android, users could request a "New identity" by interacting with a persistent notification that appeared while Tor Browser was running. However this was unusual for a couple of reasons – firstly, no such feature was available in the app's user interface itself, and it could only be enabled outside the app via Android's notification drawer. Secondly, this feature did not perform the same steps as Tor Browser for desktop to reset your identity, such as closing all tabs, clearing all cookies and wiping your browsing history. In actuality, it functioned more like a "refresh all circuits" button.

We've since learned that most Android users were relying on this feature as workaround to request a new circuit for a broken website, rather than reset their identity as such. So when this notification inadvertently disappeared in Tor Browser 13.5, we didn't rush to bring it back in its original state. Instead, we chose to focus our efforts on porting the desktop feature "New circuit for this site" to Android, allowing our mobile users to request a new circuit in a more targeted fashion.

To communicate this better I am updating the entry in the list of Known Issues to state that the "New Identity" feature of Tor Browser is not available on Android.