feat(esp_libc): make picolibc default libc

docs/en/api-guides/stdio.rst

@@ -58,13 +58,26 @@ Enabling one of these option will cause the corresponding VFS driver to be built
 Standard Streams and FreeRTOS Tasks
 -----------------------------------
 
+ESP-IDF provides two different implementations of standard I/O streams based on the selected LibC implementation defined by :ref:`CONFIG_LIBC`. The behavior of ``stdin``, ``stdout``, and ``stderr`` streams differs between these implementations, particularly regarding how they are shared across FreeRTOS tasks.
+
+Common to both implementations, each stream (``stdin``, ``stdout``, ``stderr``) has a mutex associated with it to protect the stream from concurrent access by multiple tasks. For example, if two tasks are writing to ``stdout`` at the same time, the mutex ensures that the outputs from each task are not mixed together.
+
+Newlib
+^^^^^^
+
 In ESP-IDF, to save RAM, ``FILE`` objects for ``stdin``, ``stdout``, and ``stderr`` are shared between all FreeRTOS tasks, but the pointers to these objects are unique for every task. This means that:
 
 - It is possible to change ``stdin``, ``stdout``, and ``stderr`` for any given task without affecting other tasks, e.g., by doing ``stdin = fopen("/dev/uart/1", "r")``.
 - To change the default ``stdin``, ``stdout``, ``stderr`` streams for new tasks, modify ``_GLOBAL_REENT->_stdin`` (``_stdout``, ``_stderr``) before creating the task.
 - Closing default ``stdin``, ``stdout``, or ``stderr`` using ``fclose`` closes the ``FILE`` stream object, which will affect all other tasks.
 
-Each stream (``stdin``, ``stdout``, ``stderr``) has a mutex associated with it. This mutex is used to protect the stream from concurrent access by multiple tasks. For example, if two tasks are writing to ``stdout`` at the same time, the mutex will ensure that the outputs from each task are not mixed together.
+Picolibc
+^^^^^^^^
+
+According to the POSIX standard, all default ``stdin``, ``stdout``, and ``stderr`` streams are global and shared between all FreeRTOS tasks. This means that:
+
+- Modifying ``stdin``, ``stdout``, or ``stderr`` will affect all other tasks. It is not possible to change standard I/O streams for specific tasks.
+- If a thread-local stream is needed, it should be implemented in the application code by opening a file stream and using it within tasks, e.g., ``fscanf()``, ``fprintf()``, etc.
 
 Blocking and non-blocking I/O
 -----------------------------