clarifying some flip commands

code/templates/flip

@@ -22,9 +22,10 @@ _help() {
 
   Filename:
     When reading from clipboard:
-      File or folder to write the clipboard contents. If omitted: stdout
+      File or folder to write the clipboard contents. "-" or pipe
+      output for stdout.
     When writing to clipboard:
-      File or folder to read from. If omitted: stdin
+      File or folder to read from. If omitted: stdin.
 
   Shorthand:
     echo my data | $SELF file.ext  # writes data with name: file.ext