Remove ‘bad top format’ error

This is unreachable.  It only happens in pp_leavewrite when IoTOP_GV(io)
is null, but the code leading up to it makes sure that IoTOP_GV(io) is not
null.  If it is still null, it jumps past the error with ‘goto forget_top’.
Stop /(a|b)(?=a){3}/ from warning twice

[sprout@dromedary-001 perl2.git]$ ../perl.git/Porting/bisect.pl --target=miniperl  --start=perl-5.8.0 --end=v5.10.0   -- ./miniperl -Ilib -we 'BEGIN { $SIG{__WARN__} = sub { die if $_[0] =~ /Quantifier/ && $warned++; warn shift }}; ""=~/(N|N)(?{})?/'
...
07be1b83a6b2d24b492356181ddf70e1c7917ae3 is the first bad commit
commit 07be1b83a6b2d24b492356181ddf70e1c7917ae3
Author: Yves Orton <demerphq@gmail.com>
Date:   Fri Jun 9 02:56:37 2006 +0200

    Re: [PATCH] Better version of the Aho-Corasick patch and lots of benchmarks.
    Message-ID: <9b18b3110606081556t779de698r82f361d82a05fbc8@mail.gmail.com>

    (with tweaks)

    p4raw-id: //depot/perl@28373

Since that commit, it has been possible for S_study_chunk to be called
twice if the trie optimisation kicks in (which happens for /(a|b)/).
‘Quantifier unexpected on zero-length expression’ is the only warning
in S_study_chunk.  Now it can appear twice if the quantified zero-
length expression is in the same regexp as a trie optimisation.

So pass a flag to S_study_chunk when ‘restudying’ to indicate that the
warning should be skipped.

There are two code paths that call S_study_chunk, one for when there
is no top-level alternation, which triggers the error in this case,
and one for when there is a top-level alternation (/a|b/).  I wasn’t
able to figure out how to trigger the double warning in the second
case, but I passed the flag for the restudy in that code path anyway,
since I don’t think it can be wrong.
diag.t: Remove nonexistent exception
Make ‘make regen’ regenerate the tree in perllexwarn

‘perl regen/warnings.pl tree’ would already generate the tree, but it
had to be run separately and then copied and pasted into perllexwarn.
Now regen/warnings.pl modifies perllexwarn in place as part of its
regeneration.  The ‘tree’ command line argument will still cause the
tree to be output to STDOUT.

This causes the three missing experimental categories to be listed in
perllexwarn, resolving ticket #118369.
diag.t: Remove more nonexistent exceptions
perlfunc: Document fallback to "top" format

perl 4’s perl.man says this:

By default the top‐of‐page format is "top", but it may be set to the
format of your choice by assigning the name to the $^ variable.

This is still the case, but is completely undocumented in perl 5.

In perl 5 the presence of a <FILEHANDLE_NAME>_TOP format will over-
ride "top", as will $^, but "top" is still checked for in the
absence of those.

This should be documented.  One thing I do mention here that perl 4
omits is that it is "top" in the current package.

I also tweaked the wording of the preceding sentence.
perldiag: Document ‘Can't call mro_isa_changed_in’
perldelta for #118159 (SvTRUE on ""/1 dualvar)
perldelta for 3036c853/\N{} deprecations
perldelta for deparsing for my $x (#117081)
perldelta for constant inlining (70e5f2b5)
perldelta for two lexsub fixes
perldelta for lexsub syntax errors (3a74e0e282c)
perldelta: typo
perldelta for #117311/B::Deparse -l and FF
perldelta for JS in perltrap
perldelta for deparsing ${#}{foo} (#117531)
perldelta for #117595 (-l warning)
perldelta for (?[]) fixes
perldealt for delete local $ENV{x} leaking (9332b95f7)