Improve &exit documentation.

Specify that exit status code has to be interpreted by calling process, and
that LEAVE is prevented when using exit.
Remark that &exit should be used to terminate abnormally only the main process,
while exceptions should be used for other cases (e.g., methods).

This does not remove the TODO item attached, but I believe
there's not a better place to document exit since it is already documented
with the usage of MAIN and the phasers.

Author: fluca1978

doc/Type/Any.pod6

@@ -293,7 +293,14 @@ Defined as:
 
     sub exit(Int() $status = 0)
 
-Exits the current process with return code C<$status>.
+Exits the current process with return code C<$status> or zero if no value has been specified.
+The exit value (C<$status>), when different from zero, has to be opportunely evaluated from the
+process that catches it (e.g., a shell).
+
+C<exit> does prevent the L<LEAVE|/phasers#LEAVE> phaser to be executed.
+
+C<exit> should be used as last resort only to signal the parent process about an exit code different from zero,
+and should not be used to terminate exceptionally a method or a sub: use  L<exceptions|/language/exceptions> instead.
 
 =comment TODO maybe find a better place to document &exit