Add caveat for usb_cdc.Serial.connected; fix another doc bug

[dhalbert]

Fixes #4345.
Fixes #4352.

These are documentation changes only.

[ismael-miguel]

Loosely related to issue #4345:

The usb_cdc documentation (https://circuitpython.readthedocs.io/en/latest/shared-bindings/usb_cdc/index.html) is missing the following methods :

• readline() - returns bytearray, takes 0 or 1 argument, seems to be a _typing.WriteableBuffer) or some sort of stream
• readlines() - returns array of bytearray

Aditionally, it is missing any information about how to change the newline character, if possible at all.

[jepler]

@dhalbert I think this is good to go but I'm leaving un-merged in case you want to make any change related to @ismael-miguel's review comment (and the build's not finished anyway)

[dhalbert]

Anybody can review and merge.

[jepler]

Thank you.