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

@@ -7380,7 +7380,15 @@ them.
     explicitly set, in which case it 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. Besides, on color managed
+    platforms, the output metadata will most likely to be correct. For example,
+    macOS, Windows with ACM enabled, or Wayland that compositor has color
+    management protocol support are all globally color managed.
+
+    However, your render API should also support passing output metadata to MPV,
+    or able to let MPV pick color space description for output surface. Setting
+    ``--gpu-api`` to ``d3d11`` or ``vulkan`` (``opengl`` won't work) would allow
+    this.
 
     (Only for ``--vo=gpu-next``)