mderef_u: docs and B::aux_list()

Note: mderef_u is not backportable to 5.24.1.
We changed the internal MDEREF action size from 7 to 8 bits.

Closes #126

Author: Reini Urban

ext/B/B.xs

@@ -1470,6 +1470,7 @@ aux_list(o)
                         case MDEREF_INDEX_none:
                             last = 1;
                             break;
+                        case MDEREF_INDEX_const|MDEREF_INDEX_uoob:
                         case MDEREF_INDEX_const:
                             if (is_hash)
                               PUSH_SV(++items);

pod/perlcdelta.pod

@@ -14,7 +14,7 @@ L<perl5240cdelta>, which describes differences between 5.22.3 and 5.24.0.
 
 =head1 Notice
 
-perl5.24.1 was security update to non-existing problem. cperl added
+perl5.24.1 was security update to a non-existing problem. cperl added
 PERLIO_DEBUG as optional handle to redirect DEBUGGING outout via -Dx
 switches to. This is only usable with DEBUGGING perls, which are not
 used in production, only during development.
@@ -35,6 +35,20 @@ L</Selected Bug Fixes> section.
 
 =item *
 
+The C<multideref> OP has a new C<MDEREF_INDEX_uoob> flag, which can
+only appear together with C<MDEREF_INDEX_const>. This is used for
+unchecked out-of-bounds checks for arrays, to use the previous
+AvSHAPED array optimizations with multideref OPs also.
+
+But multideref ops in loops are not yet compiled to use
+C<MDEREF_INDEX_uoob> when the index is ensured to be inside the
+array. This oob optimization is expected soon.
+
+C<MDEREF_INDEX_MASK> changed to 0x70, C<MDEREF_MASK> to 0x10F, the
+C<MDEREF_SHIFT> size from 7 to 8.
+
+=item *
+
 The new C<strQEc>/C<strNEc> macros are used instead of
 C<strEQ(s,"constant")>. This enables word-wise comparison via memcpy,
 in opposite of byte-wise comparisons via strcmp with already known