doc: add clarity to what a "fpga" image is

Update fit documentation to clarify that FPGA images are vendor specific and opaque bitstreams.

Signed-off-by: Sam Povilus <sam.povilus@amd.com>
diff --git a/doc/usage/fit/source_file_format.rst b/doc/usage/fit/source_file_format.rst
index 310225d..7727ab7 100644
--- a/doc/usage/fit/source_file_format.rst
+++ b/doc/usage/fit/source_file_format.rst
@@ -198,7 +198,7 @@
     firmware              Firmware
     firmware_ivt          Firmware with HABv4 IVT
     flat_dt               Flat Device Tree
-    fpga                  FPGA Image
+    fpga                  FPGA Device Image (bitstream file, vendor specific)
     gpimage               TI Keystone SPL Image
     imx8image             NXP i.MX8 Boot Image
     imx8mimage            NXP i.MX8M Boot Image