doc(README_json README_shell): clarify the usage of PLAT argument

- Add note clarifying the usage of <PLAT> command line argument

Co-authored-by: Farshid Tavakolizadeh <email@farshid.ws>
Signed-off-by: Lincoln Wallace <lincoln.wallace@canonical.com>

README_json.md

@@ -49,6 +49,11 @@ OPCUA-SERVER is a C-application that can transmit and receive ETH_UADP packets u
 AF_PACKET or AF_XDP sockets using libopen62541 (OPCUA-based library) APIs. It
 accepts only 1 .json.i file as input.
 
+
+**NOTE:** The `PLAT` command line argument refers to either the CPU platform 
+or the ethernet controller (e.g. the [i225](json/i225)), see in [json folder](json) 
+the available platforms.
+
 ### About: 1-port
 
 Very simple, just 1 pub/sub thread

README_shell.md

@@ -29,6 +29,10 @@ to each other via an Ethernet connection and user has a terminal open in the
 This example requires multiple pin header connections for (pulse-per-second)
 PPS and (auxiliary timestamping) AUXTS.
 
+**NOTE:** The `PLAT` command line argument refers to either the CPU platform 
+or the ethernet controller (e.g. the [i225](shell/i225)), see in [shell folder](shell) 
+the available platforms.
+
 0.  [Board A & B] Build the project.
 
     ```sh