fix(doc): clarify profile port protocol usage

Ref: #2717
Signed-off-by: dankeboy36 <[email protected]>

Author: dankeboy36

docs/sketch-project-file.md

@@ -71,7 +71,9 @@ The following fields are available since Arduino CLI 1.1.0:
 - `port_config` section with `<PORT_SETTING_NAME>` and `<PORT_SETTING_VALUE>` defines the port settings that will be
   used in the `monitor` command. Typically is used to set the baudrate for the serial port (for example
   `baudrate: 115200`) but any setting/value can be specified. Multiple settings can be set. These fields are optional.
-- `<PORT_PROTOCOL>` is the protocol for the port used to upload and monitor the board. This field is optional.
+- `<PORT_PROTOCOL>` is the protocol for the port used to upload and monitor the board. Arduino CLI ignores this value at
+  runtime, so write it only if an external client (for example one using the gRPC [`Port`](/rpc/commands/#port)) needs to restore the `protocol` together with the port `address`. Clients that do not set it should assume their
+  own default (usually `serial`). This field is optional.
 
 #### Using a system-installed platform.