Added one-sentence description for NFCController and NFCEEPROM

Author: Donatien Garnier

docs/reference/api/connectivity/nfc/NFCController.md

@@ -1,5 +1,7 @@
 ## NFC controller
 
+Using an NFC controller with Mbed OS allows you to emulate NFC tags that can be read by a smartphone and generate NDEF messages on demand.
+
 To use an NFC controller, you must initiate the instance with a driver instance, an event queue and a scratch buffer for NDEF messages.
 
 ### NFC controller class reference

docs/reference/api/connectivity/nfc/NFCEEPROM.md

@@ -1,6 +1,8 @@
 ## NFC EEPROM
 
-To use an NFC controller, you must initiate the instance with a driver instance, an event queue and a scratch buffer for NDEF messages.
+An NFC EEPROM can store NDEF messages. A smartphone can be used to access the message through the NFC interface the EEPROM exposes.
+
+To use an NFC EEPROM, you must initiate the instance with a driver instance, an event queue and a scratch buffer for NDEF messages.
 
 ### NFC EEPROM class reference