[docs] Python 3 docs on Customizing LogRecord

For Python 3, the [Customizing LogRecord](https://docs.python.org/3/howto/logging-cookbook.html#customizing-logrecord) should be placed at the beginning of the [Adding contextual information to your logging output](https://docs.python.org/3/howto/logging-cookbook.html#adding-contextual-info...) section. ___ Jiann-Ming Su Sent with [Proton Mail](https://proton.me/) secure email.