Update warning on new identity now that this feature exists in the browser

Our warning:

https://tails.boum.org/doc/about/warning/index.en.html#identities

only mentions the new identity feature of Vidalia. Now that the browser also has a new identity feature, at least the part related to cookies should be rewritten.

Feature Branch: doc/7226-torbrowser_new_identity

Parent Task: #7226 (closed)

Original created by @sajolida on 7470 (Redmine)