Update structs.go

Co-authored-by: Fedor Lapshin <fe.lap.prog@gmail.com>

structs.go

@@ -614,6 +614,7 @@ type GuildPreview struct {
 	ApproximateMemberCount int `json:"approximate_member_count"`
 
 	// Approximate number of non-offline members in this guild
+	// NOTE: this field is only filled when using GuildWithCounts
 	ApproximatePresenceCount int `json:"approximate_presence_count"`
 
 	// the description for the guild