Update security target

[infeo]

We had a FR/discussion on cryptomator about Cryptomator files should have an extension. Certain points from this discussion should be added to the security target section to make the users aware of it.

See cryptomator/cryptomator#915

[overheadhunter]

While we're at it, it's also worth mentioning, that cryptomator/cryptomator#119 is an accepted risk and a fix have unacceptable impact on I/O.