make sure that C/CPP extraflags are detailed in the migration guide.

[SuGlider]

          Sure thing! @VojtechBartoska please make sure this ends up in the migration guide. cc @P-R-O-C-H-Y @lucasssvaz

Originally posted by @me-no-dev in #9562 (comment)

Reference:
https://github.com/espressif/arduino-esp32/blob/master/platform.txt#L66-L67