Fix bug #10783 with doc strings of selection-related functions and variables.

 src/w16select.c (Fx_selection_exists_p): Sync doc string with
 xselect.c.
 lisp/term/pc-win.el (x-select-text, x-selection-owner-p)
 (x-own-selection-internal, x-disown-selection-internal)
 (x-get-selection-internal): Sync doc strings and argument lists
 with xselect.c, common-win.el and x-win.el.