Don't use begin() method to just change baudrate

[GitHubLionel]

Ok, in that case, we need a warning for the begin() method to say that if you just want to change the Baudrate you must use updateBaudRate() method even if uartBegin() seems to allow this change.

Originally posted by @GitHubLionel in #10314

[GitHubLionel]

But I think that it will be better to correct the code in esp32-hal-uart.c since uartBegin() method should be do that.

[Jason2866]

There is nothing to correct when using wrong command for changing the UART speed.
As shown in example https://github.com/espressif/arduino-esp32/blob/master/libraries/ESP32/examples/Serial/Serial_All_CPU_Freqs/Serial_All_CPU_Freqs.ino

[SuGlider]

Up on the Arduino specification, Serial.begin(baud_rate) must correctly change the baud rate.
Therefore, I consider it as a bug.

I'll get a PR to fix it.