doc/filters: link framesync options for msad/identity/vif

doc/filters.texi

@@ -15070,6 +15070,8 @@ the logging system.
 
 The filter stores the calculated identity scores of each frame in frame metadata.
 
+This filter also supports the @ref{framesync} options.
+
 In the below example the input file @file{main.mpg} being processed is compared
 with the reference file @file{ref.mpg}.
 
@@ -17202,6 +17204,8 @@ the logging system.
 
 The filter stores the calculated MSAD of each frame in frame metadata.
 
+This filter also supports the @ref{framesync} options.
+
 In the below example the input file @file{main.mpg} being processed is compared
 with the reference file @file{ref.mpg}.
 
@@ -24048,6 +24052,8 @@ The obtained average VIF score is printed through the logging system.
 
 The filter stores the calculated VIF score of each frame.
 
+This filter also supports the @ref{framesync} options.
+
 In the below example the input file @file{main.mpg} being processed is compared
 with the reference file @file{ref.mpg}.