doc/gatt-api: Update documentation regarding link option

This reflects what the code has already doing so any server operation which
requires the device option shall also get the link type as well.
diff --git a/doc/gatt-api.txt b/doc/gatt-api.txt
index 60a997c..0f1cc90 100644
--- a/doc/gatt-api.txt
+++ b/doc/gatt-api.txt
@@ -84,6 +84,7 @@
 
 			Possible options: "offset": Start offset
 					  "device": Device path (Server only)
+					  "link": Link type (Server only)
 
 			Possible Errors: org.bluez.Error.Failed
 					 org.bluez.Error.InProgress
@@ -117,6 +118,7 @@
 
 			Possible options: "device": Object Device (Server only)
 					  "MTU": Exchanged MTU (Server only)
+					  "link": Link type (Server only)
 
 			Possible Errors: org.bluez.Error.Failed
 					 org.bluez.Error.NotSupported
@@ -152,6 +154,7 @@
 
 			Possible options: "device": Object Device (Server only)
 					  "MTU": Exchanged MTU (Server only)
+					  "link": Link type (Server only)
 
 			Possible Errors: org.bluez.Error.Failed
 					 org.bluez.Error.NotSupported