doc/filters: note expr and eval mode support in overlay_cuda

Commit 58c908cf94 added support for expressions for x and y parameters.
Also added was option to set eval frequency.

doc/filters.texi

@@ -15910,9 +15910,54 @@ It accepts the following parameters:
 @table @option
 @item x
 @item y
-Set the x and y coordinates of the overlaid video on the main video.
+Set expressions for the x and y coordinates of the overlaid video
+on the main video.
+
+They can contain the following parameters:
+
+@table @option
+
+@item main_w, W
+@item main_h, H
+The main input width and height.
+
+@item overlay_w, w
+@item overlay_h, h
+The overlay input width and height.
+
+@item x
+@item y
+The computed values for @var{x} and @var{y}. They are evaluated for
+each new frame.
+
+@item n
+The ordinal index of the main input frame, starting from 0.
+
+@item pos
+The byte offset position in the file of the main input frame, NAN if unknown.
+
+@item t
+The timestamp of the main input frame, expressed in seconds, NAN if unknown.
+
+@end table
+
 Default value is "0" for both expressions.
 
+@item eval
+Set when the expressions for @option{x} and @option{y} are evaluated.
+
+It accepts the following values:
+@table @option
+@item init
+Evaluate expressions once during filter initialization or
+when a command is processed.
+
+@item frame
+Evaluate expressions for each incoming frame
+@end table
+
+Default value is @option{frame}.
+
 @item eof_action
 See @ref{framesync}.