[Tails-dev] Update Electrum documentation for Tails 1.8 upgr…

Please see https://labs.riseup.net/code/issues/9713 for context.

Recommend users to manually select a trusted .onion server to protect
against DoS after note about SPV vulnerability.

Make a note that Electrum uses mBTC as the default base unit. 1 mBTC =
0.001 BTC. It can be changed in preferences.