Better descriptions for PL_regex_pad and PL_regex_padav.

p4raw-id: //depot/perl@32944

intrpvar.h

@@ -561,9 +561,12 @@ PERLVAR(Inumeric_radix_sv,	SV *)	/* The radix separator if not '.' */
 #endif
 
 #if defined(USE_ITHREADS)
-PERLVAR(Iregex_pad,     SV**)		/* All regex objects */
-PERLVAR(Iregex_padav,   AV*)		/* All regex objects */
-
+PERLVAR(Iregex_pad,     SV**)		/* Shortcut into the array of
+					   regex_padav */
+PERLVAR(Iregex_padav,   AV*)		/* All regex objects, indexed via the
+					   values in op_pmoffset of pmop.
+					   Entry 0 is an array of IVs listing
+					   the now-free slots in the array */
 #endif
 
 #ifdef USE_REENTRANT_API