Tweak copy for speedup and cancelling transactions

Continuation of #2669

These are the description for speedup and cancelling transactions respectively. Do we want to tweak them especially the latter (because it mentions "speed up" too, not sure if it'll be confusing):

> Note that speeding up this transaction does not guarantee your original transaction will be accelerated. If your attempt is successful, you will be charged the miner fee as above.

> Note that speeding up this transaction does not guarantee your original transaction will be cancelled. If your attempt is successful, you will be charged the miner fee as above.