Merge pull request #354 from ipfs/hacdias-patch-1

docs: validity follows expiration window

ipns/IPNS.md

@@ -132,7 +132,8 @@ A logical IPNS record is a data structure containing the following fields:
   - Implementations MUST support `validityType = 0` and include this value in both `IpnsEntry.validityType` and inside the DAG-CBOR document at `IpnsEntry.data[validityType]`.
 - **Validity** (bytes)
   - When `validityType = 0`
-    - Expiration date of the record with nanoseconds precision.
+    - Expiration date of the record with nanoseconds precision.  Expiration time should match the publishing medium's window.
+      - For example, IPNS records published on the DHT should have an expiration time set to within 48 hours after publication. Setting the expiration time to longer than 48 hours will not have any effect, as DHT peers only keep records for up to 48 hours.
     - Represented as an ASCII string that follows notation from [RFC3339](https://www.ietf.org/rfc/rfc3339.txt) (`1970-01-01T00:00:00.000000001Z`).
   - Implementations MUST include this value in both `IpnsEntry.validity` and inside the DAG-CBOR document at `IpnsEntry.data[validity]`.
 - **Sequence** (uint64)