Initial doc for Thread.is-initial-thread

Author: lizmat

doc/Type/Thread.pod6

@@ -135,6 +135,22 @@ L<name|#method_name>.
     my $t = Thread.new(code => { for 1..5 -> $v { say $v }}, name => 'calc thread');
     say $t.Str;                           # OUTPUT: «Thread<3>(calc thread)␤»
 
+=head2 method is-initial-thread
+
+    method is-initial-thread(--> Bool)
+
+Returns a Bool indicating whether the current thread (if called as a class
+method) or the Thread object on which it is called, is the initial thread
+of the program seen from a HLL-language perspective.
+
+    say Thread.is-initial-thread;    # True if this is the initial thread
+    say $*THREAD.is-initial-thread;  # True if $*THREAD is the initial thread
+
+Please note there is no guarantee that this is actually the main thread from
+the OS's point of view.  Also note that if you need this other than from a
+pure introspection / debugging point of view, that there are probably better
+ways to achieve what you're trying to achieve.
+
 =head1 Routines
 
 =head2 sub full-barrier