Docs: Fix bad instructions about keystore passwords

Although it LOOKS like using a temporary password for the pkcs12 archive should
work, it will actually break. (I don't know why this is; keytool must be reusing
that password as a default value for something else when it does the import,
maybe the keypass.)
Merge pull request #149 from nfagerlund/docs_keystore_passwords

Docs: Fix bad instructions about keystore passwords