doc: FIT image: clarify usage of "compression" property

Compressed images should have their compression property
set to "none" if U-Boot should leave them compressed.

This is especially the case for compressed ramdisks that
should be uncompressed by the kernel only.
Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>

doc/uImage.FIT/source_file_format.txt

@@ -164,7 +164,9 @@ the '/images' node should have the following layout:
   - data : Path to the external file which contains this node's binary data.
   - compression : Compression used by included data. Supported compressions
     are "gzip" and "bzip2". If no compression is used compression property
-    should be set to "none".
+    should be set to "none". If the data is compressed but it should not be
+    uncompressed by U-Boot (e.g. compressed ramdisk), this should also be set
+    to "none".
 
   Conditionally mandatory property:
   - os : OS name, mandatory for types "kernel" and "ramdisk". Valid OS names