[PATCH v2 2/2] Documentation: add extlinux.conf support description

[Alexander Shiyan <eagle.alexander923@gmail.com>]

Signed-off-by: Alexander Shiyan <eagle.alexander923@gmail.com>
---
 Documentation/user/booting-linux.rst | 42 ++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/Documentation/user/booting-linux.rst b/Documentation/user/booting-linux.rst
index 3972777f30..a192efd453 100644
--- a/Documentation/user/booting-linux.rst
+++ b/Documentation/user/booting-linux.rst
@@ -309,6 +309,48 @@ Additional notes about keys in the bootloader spec entries:
    different devices without having to specify a different ``root=`` option each
    time.
 
+.. _extlinux_conf:
+
+extlinux.conf Support
+^^^^^^^^^^^^^^^^^^^^^
+
+In addition to the Boot Loader Specification, barebox also supports the
+extlinux configuration format, commonly used by the Syslinux bootloader and
+many Linux distributions. This format is often found on SD cards, USB drives,
+or disk partitions prepared with tools like ``extlinux --install``.
+
+The configuration file is named ``extlinux.conf`` and can be located at:
+* ``/boot/extlinux/extlinux.conf``
+* ``/extlinux/extlinux.conf``
+
+The file uses a simple key-value syntax with sections labeled by ``LABEL``.
+A typical example looks like:
+
+.. code-block:: none
+
+  DEFAULT linux
+  LABEL linux
+    KERNEL /boot/vmlinuz
+    INITRD /boot/initrd.img
+    FDT /boot/board.dtb
+    APPEND console=ttyS0,115200 root=PARTUUID=deadbeef-01
+
+When booting an extlinux entry, barebox strips any ``root=`` parameter from the
+``APPEND`` line **only if** the global variable ``global.bootm.appendroot`` is
+set to true. After stripping, barebox injects the correct ``root=`` based on
+the boot device (similar to the ``linux-appendroot`` feature in blspec). This
+makes the same root filesystem image usable across different storage media.
+
+To use extlinux support, enable ``CONFIG_EXTLINUX`` in your barebox
+configuration. Entries are automatically discovered by the :ref:`command_boot`
+command when scanning a device or mount point. For example:
+
+.. code-block:: sh
+
+  global.bootm.appendroot=true
+  global.boot.default=mmc1.2
+  boot
+
 .. _booting_linux_net:
 
 Network boot
-- 
2.52.0