* lisp/eshell/em-cmpl.el: Use completion-at-point i.s.o pcomplete

* lisp/eshell/em-cmpl.el: Use completion-at-point i.s.o pcomplete

(eshell-cmpl-initialize): Refrain from binding to the `tab` key,
which prevents the tab -> TAB remapping.
Use completion-at-point and completion-help-at-point.
(eshell-complete-commands-list): Use `fboundp` test instead of ugly
gymnastics to try and hide the function call from the compiler.
(eshell-pcomplete): Make it an alias of completion-at-point.

* doc/misc/eshell.texi (Completion): Change wording to reflect
different default behavior.