Change constant definition syntax so they show up in godoc

for TxAccessMode, TxDeferrableMode and TxIsoLevel

After this commit, Godoc understands these are the valid values of these
types and shows them together in the documentation.

tx.go

@@ -15,10 +15,10 @@ type TxIsoLevel string
 
 // Transaction isolation levels
 const (
-	Serializable    = TxIsoLevel("serializable")
-	RepeatableRead  = TxIsoLevel("repeatable read")
-	ReadCommitted   = TxIsoLevel("read committed")
-	ReadUncommitted = TxIsoLevel("read uncommitted")
+	Serializable    TxIsoLevel = "serializable"
+	RepeatableRead  TxIsoLevel = "repeatable read"
+	ReadCommitted   TxIsoLevel = "read committed"
+	ReadUncommitted TxIsoLevel = "read uncommitted"
 )
 
 // TxAccessMode is the transaction access mode (read write or read only)
@@ -26,8 +26,8 @@ type TxAccessMode string
 
 // Transaction access modes
 const (
-	ReadWrite = TxAccessMode("read write")
-	ReadOnly  = TxAccessMode("read only")
+	ReadWrite TxAccessMode = "read write"
+	ReadOnly  TxAccessMode = "read only"
 )
 
 // TxDeferrableMode is the transaction deferrable mode (deferrable or not deferrable)
@@ -35,8 +35,8 @@ type TxDeferrableMode string
 
 // Transaction deferrable modes
 const (
-	Deferrable    = TxDeferrableMode("deferrable")
-	NotDeferrable = TxDeferrableMode("not deferrable")
+	Deferrable    TxDeferrableMode = "deferrable"
+	NotDeferrable TxDeferrableMode = "not deferrable"
 )
 
 // TxOptions are transaction modes within a transaction block