Fix symbol completion and document it.

Fix symbol completion and document it.

* doc/misc/ses.texi (Configuring what printer function
applies): Add description of keys for completing local printer
symbols and listing local printers in a help buffer.
(Formulas): Add decription for key to list the named cell
symbols in a help buffer.

* lisp/ses.el (ses-completion-keys): New constant.
(ses--completion-table): New defvar.
(ses--list-orig-buffer): New defvar.
(ses-mode-edit-map): Fixed for symbol completion, plus add
help functions to list named cells or local printers.
(ses-edit-cell-complete-symbol)
(ses--edit-cell-completion-at-point-function): New defuns for
completion during formula edition.
(ses-edit-cell): Redefine dynamically edit keymap for
completion keys to point at the right function.
(ses-read-printer-complete-symbol)
(ses--read-printer-completion-at-point-function): New defuns
for completion during printer edition.
(ses-read-printer): Redefine dynamically edit keymap for
completion keys to point at the right function.
(ses-list-local-printers): New defun.
(ses-list-named-cells): New defun.