Better document security issues, see also #42

Based on @hashier's pull request in
https://github.com/soffes/sskeychain/pull/42 this better documents that
`AccessibilityType` really should be set. It does not change any
defaults but only updates documentation accordingly. So it is 100%
backwards compatible but should help developers to make an informed
decosion about how to store items in the keychain.