Fix the effects and documentation of 'dired-free-space'

Fix the effects and documentation of 'dired-free-space'

* lisp/dired.el (dired-free-space): Fix doc string and Custom tags.
(dired--insert-disk-space): When 'dired-free-space' is 'separate',
return the position of the beginning of the disk-space line, to be
compatible with pre-Emacs 29 behavior under
'dired-hide-details-mode'.  (Bug#65186)

* doc/emacs/dired.texi (Misc Dired Features): Fix wording in
documentation of 'dired-free-space'.