Documentation/gfx: explain port mapping in libgfxinit's config

Change-Id: Id24ded4ba641aade66468313e33ede1a82090f05
Signed-off-by: Stefan Tauner <stefan.tauner@gmx.at>
Reviewed-on: https://review.coreboot.org/27854
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/Documentation/gfx/libgfxinit.md b/Documentation/gfx/libgfxinit.md
index 472e967..2086a90 100644
--- a/Documentation/gfx/libgfxinit.md
+++ b/Documentation/gfx/libgfxinit.md
@@ -76,8 +76,11 @@
     select GFX_GMA_ANALOG_I2C_HDMI_D
 
 Beside Kconfig options, *libgfxinit* needs to know which ports are
-implemented on a board and should be probed for displays. Each
-board has to implement the package `GMA.Mainboard` with a list:
+implemented on a board and should be probed for displays. The mapping
+between the physical ports and these entries depends on the hardware
+implementation and can be recovered by testing or studying the output
+of `intelvbttool` or `intel_vbt_decode`.
+Each board has to implement the package `GMA.Mainboard` with a list:
 
     ports : HW.GFX.GMA.Display_Probing.Port_List;