make alias example about single-letter variants for long-form args

This should be the most interesting use case for parameter aliases in the CLI context.

Author: schultzdavid

doc/Language/create-cli.rakudoc

@@ -268,12 +268,13 @@ And here's the signature that produces each type of argument:
 
 As any other subroutine, C<MAIN> can define
 L<aliases|/language/signatures#Argument_aliases> for its named parameters.
+In particular, this can be used to define single-letter alternative names.
 
 =for code :method<False>
 sub MAIN(
   Str   $file where *.IO.f = 'file.dat',  #= an existing file to frobnicate
-  Int  :size(:$length) = 24,              #= length/size needed for frobnication
-  Bool :$verbose,                         #= required verbosity
+  Int  :l(:$length) = 24,                 #= length needed for frobnication
+  Bool :v(:$verbose),                     #= required verbosity
 ) {
     say $length if $length.defined;
     say $file   if $file.defined;
@@ -287,8 +288,8 @@ Usage:
   frobnicate.raku [--size|--length=<Int>] [--verbose] [<file>]
 
     [<file>]                 an existing file to frobnicate
-    --size|--length=<Int>    length needed for frobnication
-    --verbose                required verbosity
+    -l|--length=<Int>        length needed for frobnication
+    -v|--verbose             required verbosity
 =end code
 
 =head2 Named arrays