Add documentation for display-line-numbers

Add documentation for display-line-numbers

* doc/emacs/custom.texi (Init Rebinding):
* doc/emacs/modes.texi (Minor Modes): Remove references to
linum-mode.
* doc/emacs/display.texi (Display Custom): Describe the
line-number display.
(Optional Mode Line): Fix the index entry to not conflict with
that in "Display Custom".
* doc/emacs/basic.texi (Position Info): Add cross-reference to
"Display Custom", for line-number display.

* src/xdisp.c (syms_of_xdisp): <display-line-numbers>: Mention
display-line-numbers-disable in the doc string.

* lisp/cus-start.el (standard): Fix lst change.