improve some explanations

Author: schultzdavid

doc/Language/create-cli.rakudoc

@@ -131,7 +131,10 @@ sub MAIN(
     say 'Verbosity ', ($verbose ?? 'on' !! 'off');
 }
 
-With C<file.dat> present, this will work this way:
+The clause C<where *.IO.f> checks that the string C<$file> corresponds to the name of a file that actually exists.
+The part C<= 'file.dat'> specifies a default value for C<$file> in case no argument for it is given.
+
+If a file C<file.dat> is present, a call with no arguments will work this way:
 =begin code :lang<shell>
 $ raku frobnicate.raku
 24
@@ -158,12 +161,12 @@ Usage:
   frobnicate.raku [--length=<Int>] [--verbose] [<file>]
 =end code
 
-Although you don't have to do anything in your code to do this, it may still
-be regarded as a bit terse.
+Such usage messages are generated completely automatically for you.
+If they seem too terse, you can easily add more information to them.
 
 =head2 Improve usage messages with rakudoc comments
 
-But there's an easy way to make that usage
+There's an easy way to make the autogenerated usage
 message better by providing hints using pod features:
 
 =for code :method<False>