Document sub MAIN usage cares about WHY on candidates

Looking at the code it also cares about .WHYs on individual
parameters, but I don't know Pod enough to figure out how to
set those.

Author: zoffixznet

doc/Language/functions.pod6

@@ -985,6 +985,7 @@ C<MAIN> sub but the resulting exit code will always be zero (success).
 To provide an exit code different from 0,
 call L<exit|https://docs.perl6.org/routine/exit>.
 
+    #|(optional description for USAGE message)
     sub MAIN( Int :$length = 24,
                :file($data) where { .IO.f // die "file not found in $*CWD" } = 'file.dat',
                Bool :v(:$verbose) #`( either --verbose or -v ) )
@@ -1032,7 +1033,10 @@ If no multi candidate of C<MAIN> is found for the given command line
 parameters, the sub C<USAGE> is called. If no such method is found,
 the compiler will output a default generated usage message.
 
-    sub MAIN(Int $i){ say $i == 42 ?? 'answer' !! 'dunno' }
+    #|(is it the answer)
+    multi MAIN(Int $i){ say $i == 42 ?? 'answer' !! 'dunno' }
+    #|(divide two numbers)
+    multi MAIN($a, $b){ say $a/$b }
 
     sub USAGE(){
     print Q:c:to/EOH/;
@@ -1043,7 +1047,9 @@ the compiler will output a default generated usage message.
     }
 
 The default usage message is available inside C<sub USAGE> via read-only
-C<$*USAGE> variable.
+C<$*USAGE> variable. It will be generated based on available C<sub MAIN>
+candidates and their parameters. You can specify additional extended
+description for each candidate using C<#|(...)> Pod block to set L<WHY>.
 
 =end pod