[Perl/perl5] dbb22d: Document XSPROTO

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: dbb22d67806a4b7a54e401a9d749539c26d72408
      
https://github.com/Perl/perl5/commit/dbb22d67806a4b7a54e401a9d749539c26d72408
  Author: Karl Williamson <k...@cpan.org>
  Date:   2020-10-06 (Tue, 06 Oct 2020)

  Changed paths:
    M XSUB.h

  Log Message:
  -----------
  Document XSPROTO