man: document --max-brightness

Signed-off-by: Yuxuan Shui <[email protected]>

man/picom.1.asciidoc

@@ -256,6 +256,9 @@ May also be one of the predefined kernels: `3x3box` (default), `5x5box`, `7x7box
 *--no-ewmh-fullscreen*::
 	Do not use EWMH to detect fullscreen windows. Reverts to checking if a window is fullscreen based only on its size and coordinates.
 
+*--max-brightness*::
+	Dimming bright windows so their brightness doesn't exceed this set value. Brightness of a window is estimated by averaging all pixels in the window, so this could comes with a performance hit. Setting this to 1.0 disables this behaviour. Requires *--use-damage* to be disabled. (default: 1.0)
+
 FORMAT OF CONDITIONS
 --------------------
 Some options accept a condition string to match certain windows. A condition string is formed by one or more conditions, joined by logical operators.