doc: add "Bonded" flag to dbus property

Bonded flag is used to indicate the link key or ltk of the remote
device has been stored.

Reviewed-by: Sonny Sasaka <sonnysasaka@chromium.org>
Reviewed-by: Yun-Hao Chung <howardchung@chromium.org>
diff --git a/doc/device-api.txt b/doc/device-api.txt
index 4e824d2..6a90664 100644
--- a/doc/device-api.txt
+++ b/doc/device-api.txt
@@ -169,7 +169,17 @@
 
 		boolean Paired [readonly]
 
-			Indicates if the remote device is paired.
+			Indicates if the remote device is paired. Pairing is
+			the process where devices exchange the information to
+			establish an encrypted connection.
+
+		boolean Bonded [readonly]
+
+			Indicates if the remote device is bonded. Bonded means
+			the link key or the ltk from the pairing process has
+			been stored.
+			A PropertiesChanged signal indicate changes to this
+			status.
 
 		boolean Connected [readonly]