Supported SoftwareSerial pins for Portenta C33 are undocumented

[per1234]

On some boards, not all pins can be used with the SoftwareSerial library. This is explained in the documentation for the library, but the information for the Portenta C33 board is missing.

There is information about supported pins in the library's "SerialEcho" example sketch. However, I don't think it is trustworthy (https://github.com/arduino/ArduinoCore-renesas/issues/291).

---

Related: https://github.com/arduino/docs-content/pull/1884

[jcarolinares]

@per1234 we have an internal procedure for request and problems with docs as you may know, please follow it and your request will finish in an appropriate backlog where will be studied, thanks!