Document %*SUB-MAIN-OPTS

Closes #1397
Rakudo impl: rakudo/rakudo@40b0169d08
Spec:        Raku/roast@754a45c006

doc/Language/functions.pod6

@@ -874,6 +874,30 @@ call L<exit|https://docs.perl6.org/routine/exit>.
         exit 1;
     }
 
+=head2 C<%*SUB-MAIN-OPTS>
+
+It's possible to alter how arguments are processed before they're passed
+to C<sub MAIN {}> by setting options in C<%*SUB-MAIN-OPTS> hash. Available
+options are:
+
+=head3 C<named-anywhere>
+
+    %*SUB-MAIN-OPTS<named-anywhere> = True;
+    sub MAIN ($a, $b, :$c, :$d) {
+        say "Accepted!"
+    }
+
+By default, named arguments to the program cannot be given after any positional
+arguments are specified. However, if C«%*SUB-MAIN-OPTS<named-anywhere>» is
+set to a truthy value, named arguments can be specified anywhere, even after
+positional parameter. For example, the above program can be called with:
+
+=begin code :skip-test
+
+    perl6 example.p6 1 --c=2 3 --d=4
+
+=end code
+
 =head1 C<sub USAGE>
 X<|USAGE>