doc AM_PATH_LISPDIR versus prefix

[Kevin Ryde]

In automake 1.9 (debian package) manual, the "Public macros" node on
AM_PATH_LISPDIR says

     Searches for the program `emacs', and, if found, sets the output
     variable `lispdir' to the full path to Emacs' site-lisp
     directory.

This reads like lispdir will be the emacs path, like /usr,
irrespective of what your own package's configured --prefix is.

But if I'm not mistaken the macro in fact simply chooses between

        $libdir/emacs/site-lisp
        $libdir/xemacs/site-lisp
        $datadir/emacs/site-lisp
        $datadir/xemacs/site-lisp

according to what flavour and setup $EMACS has.  Ie. it's always under
your $prefix, just in a varying location there.  Perhaps the
documentation could be reworded to make this clear.

Incidentally, is $libdir only for old versions or emacs, and/or old
style filesystem layouts?  If that's so maybe that could be noted, or
describe $datadir/x?emacs/site-lisp first and then the $libdir second,
or something like that, so current conventions are shown first, if you
know what I mean.