document $*EXECUTABLE and note caveats of $*EXECUTABLE_NAME

Author: FROGGS

lib/variables.pod

@@ -387,6 +387,10 @@ Other compile-time constants:
     $*PID              Process ID of the current process.
     $*OS               Which Operating System am I compiling under (e.g. Linux).
     $*OSVER            Version of the current Operating System.
+    $*EXECUTABLE       Absolute path of the perl executable that is currently running.
     $*EXECUTABLE_NAME  The name of the perl executable that is currently running.
+                       (e.g. perl6-p, perl6-m, Niecza.exe)
+                       Favor $*EXECUTABLE because it is not guaranteed that the perl
+                       executable is in PATH.
 
 =end pod