Document arguments to Eshell's built-in commands

Document arguments to Eshell's built-in commands

* lisp/eshell/em-unix.el (eshell/ln): LINK_NAME is required.

* lisp/eshell/esh-ext.el (eshell/addpath):
* lisp/eshell/esh-var.el (eshell/env): Improve help strings slightly.

* doc/misc/eshell.texi (Scripts): Explain $0, $1, etc.
(Dollars Expansion): Use "@dots{}" instead of "...".
(Built-ins, Tramp extensions, Extra built-in commands): Document
command-line arguments.

(cherry picked from commit 5c43ef86bf169a79b87bd082d2f884757f7c2efc)