Update doc for unique=field

Author: shihanng

doc.go

@@ -587,9 +587,15 @@ Unique
 
 For arrays & slices, unique will ensure that there are no duplicates.
 For maps, unique will ensure that there are no duplicate values.
+For slices of struct, unique will ensure that there are no duplicate values
+in a field of the struct specified via a parameter.
 
+	// For arrays, slices, and maps:
 	Usage: unique
 
+	// For slices of struct:
+	Usage: unique=field
+
 Alpha Only
 
 This validates that a string value contains ASCII alpha characters only