add support for texinfo parsed arguments, like @acronym

add support for texinfo parsed arguments, like @acronym

* module/texinfo.scm (texi-command-specs): Add a new kind of texinfo
  command, inline-text-args, a sort of a cross between inline-args,
  which are unparsed, and inline-text, which is.  Perhaps this should
  supersede inline-args at some point.  In any case, add acronym as an
  inline-text-args element.
  (inline-content?, arguments->attlist, complete-start-command)
  (parse-inline-text-args, make-dom-parser): Adapt for
  inline-text-args.

* module/texinfo/serialize.scm (inline-text-args): Add serialization for
  @acronym.

* test-suite/tests/texinfo.test ("test-texinfo->stexinfo"): Add some
  tests.