perlrun/debguts: Connect -dt and PERL5DB_THREADS a bit better

This was going to cover more, but it turned out to already
be fairly well covered.

Author: tonycoz

pod/perldebguts.pod

@@ -34,6 +34,12 @@ Perl inserts the contents of C<$ENV{PERL5DB}> (or C<BEGIN {require
 
 =item *
 
+If the C<-dt> switch is supplied, C<PERL5DB_THREADED=1> is set in the
+environment.  C<perl5db.pl> or your custom debugger can use this to
+enable threading support.
+
+=item *
+
 Each array C<@{"_<$filename"}> holds the lines of $filename for a
 file compiled by Perl.  The same is also true for C<eval>ed strings
 that contain subroutines, or which are currently being executed.

pod/perlrun.pod

@@ -365,8 +365,9 @@ X<-d> X<-dt>
 =item B<-dt>
 
 runs the program under the Perl debugger.  See L<perldebug>.
-If B<t> is specified, it indicates to the debugger that threads
-will be used in the code being debugged.
+If B<t> is specified, C<PERL5DB_THREADED=1> is set in the environment
+to indicate to the debugger that threads will be used in the code
+being debugged.
 
 =item B<-d:>I<MOD[=bar,baz]>
 X<-d> X<-dt>