Update description in Snowflake extension pages on Firefox and Chrome

There was a discussion in the Tor IRC channel that the description in the Snowflake extension Chrome webstore and Firefox add-ons page does not clearly distinguish between censored/uncensored users:

• https://addons.mozilla.org/en-GB/firefox/addon/torproject-snowflake/
• https://chrome.google.com/webstore/detail/snowflake/mafpmfcccpbjnhfhjnllmmalhifmlcie

Opening the issue to say that I could work on updating the description in the next hour if the priority is high!

cc: @arma @gus @shelikhoo @meskio