doc: Add a description for bootmeth_qfw

Add documentation for the qfw bootmeth.

Fix up the compatible string to drop the 'extlinux' part, which is not
relevant to this 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/index.rst b/doc/develop/bootstd/index.rst
index 5052afe..f8fce72 100644
--- a/doc/develop/bootstd/index.rst
+++ b/doc/develop/bootstd/index.rst
@@ -9,3 +9,4 @@
    overview
    extlinux
    pxelinux
+   qfw