Consider adding explanation about the Master Password

A user entered today on the irc asking how could they add a master password to the Tor Browser. It seems a common doubt for new users.

Proposal:

• Maybe it will be good to have an official explanation on our Support portal about why there is no master password in Tor Browser?

From the Tor Browser Design Draft (https://2019.www.torproject.org/projects/torbrowser/design/):

We disable the password saving functionality in the browser as part of our Disk Avoidance requirement. However, since users may decide to re-enable disk history records and password saving, we also set the signon.autofillForms preference to false to prevent saved values from immediately populating fields upon page load. Since JavaScript can read these values as soon as they appear, setting this preference prevents automatic linkability from stored passwords.

(we should write a more simple answer)