Add documentation to KVPair

Based on discussions with @slackpad, some of the values in the KVPair are
ReadOnly. This commit updates the docs to reflect that.

api/kv.go

@@ -11,12 +11,32 @@ import (
 
 // KVPair is used to represent a single K/V entry
 type KVPair struct {
+	// Key is the name of the key. It is also part of the URL path when accessed
+	// via the API.
 	Key string
+
+	// CreateIndex holds the index corresponding the creation of this KVPair. This
+	// is a read-only field.
 	CreateIndex uint64
+
+	// ModifyIndex is used for the Check-And-Set operations.
 	ModifyIndex uint64
+
+	// LockIndex holds the index corresponding to a lock on this key, if any. This
+	// is a read-only field.
 	LockIndex uint64
+
+	// Flags are any user-defined flags on the key. It is up to the implementer
+	// to check these values, since Consul does not treat them specially.
 	Flags uint64
+
+	// Value is the value for the key. This can be any value, but it will be
+	// base64 encoded upon transport.
 	Value []byte
+
+	// Session is a string representing the name of the session. Any other
+	// interactions with this key over the same session must specify the same
+	// session name.
 	Session string
 }