perlsub: Improve the "Constant Functions" documentation

perlsub: Improve the "Constant Functions" documentation

Ever since perl-5.003_95-19-g5431012 which incorporated this patch:

      Title:  "Improve and update documentation of constant subs"
       From:  Tom Phoenix <rootbeer@teleport.com>
     Msg-ID:  <Pine.GSO.3.96.970331122546.14185C-100000@kelly.teleport.com>
       Date:  Mon, 31 Mar 1997 13:05:54 -0800 (PST)
      Files:  pod/perlsub.pod

We've documented that "BEGIN { my $x = 1; *X = sub () { $x } }" creates
an inlined subroutine, we just weren't doing so very explicitly. Patch
the documentation so that we now have explicit examples for the behavior
mentioned in the first paragraph of the section.

This also adds an explicit mention of the behavior reported as a bug in
[RT #79908], shows the user that you can just use B::Deparse rather than
relying on a warning to see the effects of inlining, and replaces a
contrived way to avoid inlining with just adding a "return".

While writing these docs I found a bug in the warning behavior mentioned
here, which I filed as [RT #121841]