doc: rework synopsis section for ff* tools

Fix syntax, remove pointless introductory sentence, improve overall
consistency.

doc/ffmpeg.texi

@@ -11,13 +11,7 @@
 
 @chapter Synopsis
 
-The generic syntax is:
-
-@example
-@c man begin SYNOPSIS
-ffmpeg [global options] [[infile options][@option{-i} @var{infile}]]... @{[outfile options] @var{outfile}@}...
-@c man end
-@end example
+ffmpeg [@var{global_options}] @{[@var{input_file_options}] -i @file{input_file}@} ... @{[@var{output_file_options}] @file{output_file}@} ...
 
 @chapter Description
 @c man begin DESCRIPTION

doc/ffplay.texi

@@ -11,11 +11,7 @@
 
 @chapter Synopsis
 
-@example
-@c man begin SYNOPSIS
-ffplay [options] [@file{input_file}]
-@c man end
-@end example
+ffplay [@var{options}] [@file{input_file}]
 
 @chapter Description
 @c man begin DESCRIPTION

doc/ffprobe.texi

@@ -11,13 +11,7 @@
 
 @chapter Synopsis
 
-The generic syntax is:
-
-@example
-@c man begin SYNOPSIS
-ffprobe [options] [@file{input_file}]
-@c man end
-@end example
+ffprobe [@var{options}] [@file{input_file}]
 
 @chapter Description
 @c man begin DESCRIPTION

doc/ffserver.texi

@@ -11,13 +11,7 @@
 
 @chapter Synopsys
 
-The generic syntax is:
-
-@example
-@c man begin SYNOPSIS
-ffserver [options]
-@c man end
-@end example
+ffserver [@var{options}]
 
 @chapter Description
 @c man begin DESCRIPTION