Delete duplicate docstrings in PGTK build

Delete duplicate docstrings in PGTK build

* src/pgtkfns.c (Fx_export_frames, Fx_create_frame)
(Fx_server_max_request_size, Fx_display_screens, Fx_display_mm_height)
(Fx_display_mm_width, Fx_display_backing_store, Fx_display_visual_class)
(Fx_display_save_under, Fx_open_connection, Fx_close_connection)
(Fx_display_list, Fxw_color_defined_p, Fxw_color_values)
(Fxw_display_color_p, Fx_display_grayscale_p, Fx_display_pixel_width)
(Fx_display_pixel_height, Fx_display_planes, Fx_display_color_cells)
(Fx_show_tip, Fx_hide_tip, Fx_file_dialog, Fx_select_font): Replace
duplicate docstrings with "SKIP" marker.  See the comment in doc.c.
* src/xfns.c (Fx_server_max_request_size, Fx_display_screens)
(Fx_display_backing_store, Fx_display_visual_class, Fx_file_dialog):
Update docstrings with details about the PGTK build.

(cherry picked from commit 4fcba8456e897cc77d1825e696aacb446ce45405)