doc: Add a description for bootmeth_extlinux

Add documentation for the extlinux bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
diff --git a/doc/develop/bootstd/overview.rst b/doc/develop/bootstd/overview.rst
index e04c683..26b7985 100644
--- a/doc/develop/bootstd/overview.rst
+++ b/doc/develop/bootstd/overview.rst
@@ -414,9 +414,9 @@
 Available bootmeth drivers
 --------------------------
 
-Bootmeth drivers are provided for:
+Bootmeth drivers are provided for booting from various media:
 
-   - extlinux / syslinux boot from a disk
+   - :doc:`extlinux / syslinux <extlinux>` boot from a storage device
    - extlinux boot from a network (PXE)
    - U-Boot scripts from disk, network or SPI flash
    - EFI boot using bootefi from disk