docs/install/e6400.md: Make note of 1440x900 panel errata

Due to an issue in libgfxinit, Latitude E6400 systems with a 1440 x 900
display panel would have garbled graphics before the OS boots. Make a
note of this issue in releases 20240504 and earlier.

Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>

site/docs/install/e6400.md

@@ -62,6 +62,19 @@ Intel GPU: libre video initialisation available
 Canoeboot uses coreboot's native `libgfxinit` on this platform, for
 variants with Intel graphics.
 
+Intel GPU errata
+----------------
+
+Systems with a 1440 x 900 display panel instead of the more common 1280 x 800
+panel will have garbled graphics before the OS boots (i.e. in SeaBIOS or GRUB)
+in Libreboot 20240504 and earlier. This is fixed in releases after 20240504.
+
+This was caused by libgfxinit calculating PLL divider values for the pixel clock
+assuming a 96 MHz reference frequency, whereas the E6400 uses a 100 MHz
+reference frequency. The error is not large enough to affect the lower
+resolution panels, but is enough to affect the 1440 x 900 panels which use a
+higher pixel clock.
+
 How to flash internally (no diassembly)
 =======================================