Say how to override a primitive interactive spec

Say how to override a primitive interactive spec

* doc/lispref/internals.texi (Writing Emacs Primitives): Mention that
  the `interactive-form' property can be used to override a primitive
  interactive specification, and refer to the detailed documentation
  for setting that property.

From this thread on Emacs Devel:

  https://lists.gnu.org/archive/html/emacs-devel/2018-03/msg00923.html
  From: Eli Zaretskii
  To: Karl Fogel
  CC: Juri Linkov, Emacs Devel
  Subject: Re: [Emacs-diffs] \
           master b88e7c8: Make transpose-regions interactive (Bug#30343)
  Date: Thu, 29 Mar 2018 14:38:15 +0300
  Message-Id: <834lkzdsd4.fsf@gnu.org>