fix: Rename grp dev type

ref: #1928

docs/companion_protocol.md

@@ -295,7 +295,7 @@ Bytes 4+: Binary payload bytes (variable length)
 
 **Data Type / Transport Mapping**:
 - `0x0000` is invalid for this command.
-- `0xFFFF` (`DATA_TYPE_CUSTOM`) is the generic custom-app namespace.
+- `0xFFFF` (`DATA_TYPE_DEV`) is the developer namespace for experimenting and developing apps.
 - Other non-zero values can be used as assigned application/community namespaces.
 
 **Note**: Applications that need a timestamp should encode it inside the binary payload.

src/helpers/TxtDataHelpers.h

@@ -6,7 +6,7 @@
 #define TXT_TYPE_PLAIN          0      // a plain text message
 #define TXT_TYPE_CLI_DATA       1      // a CLI command
 #define TXT_TYPE_SIGNED_PLAIN   2      // plain text, signed by sender
-#define DATA_TYPE_CUSTOM        0xFFFF // generic custom app namespace for group/channel datagrams
+#define DATA_TYPE_DEV           0xFFFF // developer namespace for experimenting with group/channel datagrams and building apps
 
 class StrHelper {
 public: