Address several comments

Author: ribasushi

UNIXFS.md

@@ -61,23 +61,16 @@ message Data {
 	optional uint64 hashType = 5;
 	optional uint64 fanout = 6;
 	optional uint32 mode = 7;
-	optional TimSpec mtime = 8;
+	optional TimeSpec mtime = 8;
 }
 
 message Metadata {
 	optional string MimeType = 1;
 }
 
 message TimeSpec {
-	// Use of int64 is deliberate - negative epoch is super unlikely
-	// Using sint64 would introduce zig-zag encoding ( harder to eyeball )
-	// The varint representing the time of writing this is 5 bytes long
-	// It will remain so until October 26, 3058 ( 34,359,738,367 )
 	required int64 EpochSeconds = 1;
 
-	// fixed32 ( always 4 bytes, no varint ), as nanosecs are often > 2^28
-	// https://developers.google.com/protocol-buffers/docs/proto#scalar
-	// NOTE: on the wire this value will be *little* endian
 	optional fixed32 EpochNanoseconds = 2;
 }
 ```
@@ -207,6 +200,21 @@ This scheme would see metadata stored in an external database.
 
 The downsides to this are that metadata would not be transferred from one node to another when syncing as [Bitswap] is not aware of the database, and in-tree metadata
 
+### TimeSpec protobuf datatype rationale
+
+#### EpochSeconds
+
+The integer portion of the epoch is represented on the wire using a varint encoding. While this is inefficient for
+negative values, it avoids introducing zig-zag encoding. Negative epoch values will be exceedingly rare, and there
+could very well be value in having such cases stand out, while at the same keeping the "usual" positive values easy
+to eyeball. The varint representing the time of writing this text is 5 bytes long. It will remain so until
+October 26, 3058 ( 34,359,738,367 )
+
+#### EpichNanoseconds
+Since fractional values will very often be > 2^28 nanoseconds, that part is represented as a 4-byte `fixed32`,
+[as per google's recommendation](https://developers.google.com/protocol-buffers/docs/proto#scalar).
+
+
 [multihash]: https://tools.ietf.org/html/draft-multiformats-multihash-00
 [CID]: https://docs.ipfs.io/guides/concepts/cid/
 [Bitswap]: https://github.com/ipfs/specs/blob/master/BITSWAP.md