Documenting LEAVE trap, closes #3148

doc/Language/traps.pod6

@@ -1279,6 +1279,25 @@ sub explicitly-return-ret () {
 }
 =end code
 
+=head2 C<LEAVE> needs explicit return from a sub to run
+
+As the documentation for the
+L<C<LEAVE> phaser indicates|/language/phasers#index-entry-Phasers__LEAVE-LEAVE>
+C<LEAVE> runs when a block is exited, "... except when the program exits
+abruptly". That is, unlike C<END>, it's going to be invoked only if the block
+actually returns in an orderly way. This is why:
+
+=for code
+sub a() { LEAVE say "left"; exit 1 }; # No output, it will simply exit
+
+will not run the C<LEAVE> code, since technically it's not returning. On the
+other hand, L<C<END>|/language/phasers#END> is a program execution phaser and
+will run no matter what
+
+=for code
+sub a() {
+    END say "Begone"; exit 1
+}; a; # OUTPUT: «Begone␤»
 
 =head1 Input and output