Improve help message in Retrieve Password form

A private key is used to decrypt a password.
It is better to update the message to mention "private key" explicitly
instead of "key pair". "key pair" is a pair of private and public key.

Change-Id: Icd5aa8fae2a348df011861469d028903bd71ec02
Closes-Bug: #1291529
Merge "Improve help message in Retrieve Password form"