Patch #940026: Explain "in" when it is first used.

Backported to 2.3.

Doc/tut/tut.tex

@@ -1428,6 +1428,9 @@ This function can be called either like this:
 \code{ask_ok('Do you really want to quit?')} or like this:
 \code{ask_ok('OK to overwrite the file?', 2)}.
 
+This example also introduces the \keyword{in} keyword. This tests
+whether or not a sequence contains a certain value.
+
 The default values are evaluated at the point of function definition
 in the \emph{defining} scope, so that