Add BEGIN, UNITCHECK, CHECK, INIT and END section

This was completely missing from the 5 -> 6 migration docs, afaics

Author: lizmat

doc/Language/5to6-nutshell.pod6

@@ -1126,6 +1126,43 @@ As with Perl 5, comments work as usual in regexes.
 
     / word #`(match lexical "word") /
 
+=head1 BEGIN, UNITCHECK, CHECK, INIT and END
+
+Except for C<UNITCHECK>, all of these special blocks exist in Perl 6 as well.
+In Perl 6, these are called L<Phasers|/language/phasers>.   But there are some
+differences!
+
+=head2 UNITCHECK becomes CHECK
+
+There is currently B<no> direct equivalent of C<CHECK> blocks in Perl 6.
+The C<CHECK> phaser in Perl 6 has the same semantics as the C<UNITCHECK>
+block in Perl 5: it gets run whenever the compilation unit in which it
+occurs, has finished parsing.  This is considered a much saner semantic
+than the current semantics of C<CHECK> blocks in Perl 5.  But for
+compatibility reasons, it was impossible to change the semantics of C<CHECK>
+blocks in Perl 5, so a C<UNITCHECK> block was introduced in 5.10.  So it
+was decided that the Perl 6 C<CHECK> phaser would follow the saner Perl 5
+C<UNITCHECK> semantics.
+
+=head2 No block necessary
+
+In Perl 5, these special blocks B<must> have curly braces, which implies a
+separate scope.  In Perl 6 this is not necessary, allowing these special
+blocks to share their scope with the surrounding lexical scope.
+
+=for code :lang<perl5>
+my $foo;             # Perl 5
+BEGIN { $foo = 42 }
+=for code
+BEGIN my $foo = 42;  # Perl 6
+
+=head2 Changed semantics with regards to precompilation
+
+If you put C<BEGIN> and C<CHECK> phasers in a module that is being
+precompiled, then these phasers will B<only> be executed during precompilation
+and B<not> when a precompiled module is being loaded.  So when porting module
+code from Perl 5, you may need to change C<BEGIN> and C<CHECK> blocks to
+C<INIT> blocks to ensure that they're run when loading that module.
 
 =head1 Pragmas