Improve the doc of eshell-cmpl-* custom variables (Bug#25069)
* lisp/eshell/em-cmpl.el (eshell-cmpl--custom-variable-docstring): New
function to generate the docstring for custom variables derived from
pcomplete-* variables.
(eshell-cmpl-file-ignore, eshell-cmpl-dir-ignore)
(eshell-cmpl-ignore-case, eshell-cmpl-autolist)
(eshell-cmpl-suffix-list, eshell-cmpl-recexact)
(eshell-cmpl-man-function, eshell-cmpl-compare-entry-function)
(eshell-cmpl-expand-before-complete, eshell-cmpl-cycle-completions)
(eshell-cmpl-cycle-cutoff-length, eshell-cmpl-restore-window-delay)
(eshell-command-completion-function, eshell-cmpl-command-name-function)
(eshell-default-completion-function, eshell-cmpl-use-paring): Use it
to set the docstring.