add docs for swift variables

Adds docs for the new swift_hash prefix and suffix flags
introduced in havana

Also reveal_sensitive_prefix - introduced in havana

Change-Id: I2ccd7a2c1216a252c251b47e9f7ac8f6f8fc89a7
Closes-Bug: 1195556
Closes-Bug: 1204119
Merge "add docs for swift variables"