Add stdout/stderr outputs to FAQ (#4175)

* Add stdout/stderr to faq

* Crosslink

Author: AbrilRBS

knowledge/faq.rst

@@ -222,3 +222,40 @@ The general best practices are:
 - Keep the versions aligned. If using a version range try to use the same version range everywhere.
 - Declare first dependencies that use fixed version, not version ranges
 - Use the ``conan graph info ... --format=html > graph.html`` graphical interactive output to understand and navigate conflicts.
+
+.. _faq_stdout_stderr_redirects:
+
+Conan is redirecting its output to stderr
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+As explained :ref:`in the commands reference<commands_output>`,
+by design Conan redirects its logging information to the standard error output (stderr),
+while the actual results of the commands are sent to the standard output (stdout).
+
+This is done to allow users to easily redirect the output of Conan commands to files or other
+processes without having the logging information mixed with the actual results.
+
+For example, running a command like:
+
+.. code-block:: text
+
+   $ conan graph info --requires=zlib/1.3.1 --format=json > graph.json
+
+   ...
+
+   ======== Computing dependency graph ========
+   Graph root
+       cli
+   Requirements
+       zlib/1.3.1#b8bc2603263cf7eccbd6e17e66b0ed76 - Cache
+
+   ======== Computing necessary packages ========
+   Connecting to remote 'conancenter' anonymously
+   Requirements
+       zlib/1.3.1#b8bc2603263cf7eccbd6e17e66b0ed76:dbb40f41e6e9a5c4a9a1fd8d9e6ccf6d92676c92#8976086f07d37e3f6288e2fccf9650ae - Cache
+
+will create a file named ``graph.json`` with the JSON output of the command, but it will not
+include any logging information, which will be printed to the console.
+
+Note that this approach is common in many command-line tools such as ``git`` and ``curl``,
+and it is not specific to Conan.

reference/commands.rst

@@ -101,6 +101,8 @@ common practices in many CLI tools and the `POSIX standard
 - ``stderr``: For diagnostic output, including logs, warnings, errors, and progress
   messages.
 
+More info can be found more in the :ref:`FAQ section<faq_stdout_stderr_redirects>`.
+
 **Redirecting Output to Files**
 
 You can redirect Conan output to files using shell redirection: