[Perl/perl5] adef08: Document NV to int cast macros

[Karl Williamson via perl5-changes]

  Branch: refs/heads/smoke-me/khw-pr_api2
  Home:   https://github.com/Perl/perl5
  Commit: adef08b0a254e15c894ee842199d2fecc54c55fb
      
https://github.com/Perl/perl5/commit/adef08b0a254e15c894ee842199d2fecc54c55fb
  Author: Karl Williamson <k...@cpan.org>
  Date:   2020-09-29 (Tue, 29 Sep 2020)

  Changed paths:
    M perl.h

  Log Message:
  -----------
  Document NV to int cast macros