[docs,qemu] Document QEMU logging

Add a short section on the QEMU tessting documentation that describes
how the QEMU UART console output and log output are muxed into stdout
without line buffering, to avoid confusion about interleaving of
characters from both inputs.

Signed-off-by: Alex Jones <[email protected]>

Author: AlexJones0

third_party/qemu/README.md

@@ -53,3 +53,10 @@ For example, to manually add some traces for the USB device and alert updates wh
 ```
 ./bazelisk.sh test //path_to_qemu_test:my_test --test_arg=--qemu-args="--trace ot_usbdev* --trace ot*update_alert*"
 ```
+
+### Logging
+
+When executing a test in a QEMU execution environment, the default behavior is that the QEMU log output stream is combined with the regular UART console output stream.
+When both interfaces are simultaneously written, it is possible that output may be disrupted by the characters interleaved from both streams.
+This can be particularly noticeable when tracing, where the QEMU log is often written much faster than regular console output.
+These outputs are not currently buffered by line breaks before combining the streams, meaning that it is possible for one line of console output to be split across multiple QEMU log lines.