Document unit sub MAIN, index existing unit and module declarators.

Closes #1819.

Author: cfa

doc/Language/functions.pod6

@@ -1048,6 +1048,20 @@ positional parameter. For example, the above program can be called with:
 perl6 example.p6 1 --c=2 3 --d=4
 =end code
 
+=head2 X<Unit-scoped definition of MAIN|declarator,unit (MAIN)>
+
+If the entire script body resides within C<MAIN>, you can use the C<unit>
+declarator as follows:
+
+    unit sub MAIN( Int :$length = 24,
+                    :file($data) where { .IO.f // die "file not found in $*CWD" } = 'file.dat',
+                    Bool :v(:$verbose) #`( -verbose, --verbose, -v or --v ) );
+
+    # rest of script is part of MAIN
+
+Note that this is only appropriate if you do not need a C<proto> or C<multi> definition.
+
+
 =head1 sub USAGE
 
 X<|USAGE>X<|$*USAGE>

doc/Language/syntax.pod6

@@ -609,7 +609,7 @@ You can also assign subroutines to variables.
     my &f = -> { say "Hello!" }  # Lambda style syntax. The & sigil indicates the variable holds a function
     my $f = -> { say "Hello!" }  # Functions can also be put into scalars
 
-=head3 Module, Class, Role, and Grammar declaration
+=head3 X<Module, Class, Role, and Grammar declaration|declarator,unit;declarator,module>
 
 There are several types of compilation units (packages), each declared with a
 keyword, a name, some optional traits, and a body of functions.
@@ -622,7 +622,7 @@ keyword, a name, some optional traits, and a body of functions.
 
     grammar Baz { }
 
-You can declare a unit of things without explicit curly brackets.
+You can declare a C<unit> of things without explicit curly brackets.
 
     =begin code :skip-test
     unit module Gar;