DOCS/man/options: add some description for sdr-adjust-gamma

Add some description about output metadata.

Signed-off-by: Shengyu Qu <[email protected]>

Author: Headcrabed

DOCS/man/options.rst

@@ -7381,7 +7381,9 @@ them.
     should always behaves like ``yes``.
 
     Generally it's recommended to enable this option, if you can ensure that
-    both source and target metadata is correct.
+    both source and target metadata is correct. On color-managed platforms, the
+    target metadata is likely to be accurate. However, the accuracy of the
+    source metadata might not be guaranteed.
 
     (Only for ``--vo=gpu-next``)