Yet more syncing of xselect docs and argument specs

Yet more syncing of xselect docs and argument specs

* src/xselect.c (Fx_own_selection_internal)
(Fx_get_selection_internal, Fx_disown_selection_internal)
(Fx_selection_owner_p, Fx_selection_exists_p): Doc fixes.
* src/nsselect.m (Fx_own_selection_internal)
(Fx_disown_selection_internal, Fx_selection_exists_p)
(Fx_selection_owner_p, Fx_get_selection_internal):
Sync docs and argument specs with the xselect.c versions.
* lisp/term/pc-win.el (x-selection-owner-p, x-own-selection-internal)
(x-disown-selection-internal, x-get-selection-internal):
Sync docs with the xselect.c versions.