Define explicit-shell-file-name only in shell.el

Define explicit-shell-file-name only in shell.el

For discussion, see the following thread:
https://lists.gnu.org/r/emacs-devel/2020-04/msg00880.html

* doc/emacs/cmdargs.texi (General Variables): Mention
shell-file-name in relation to SHELL.
* doc/emacs/misc.texi (Interactive Shell): Move index entry for
SHELL environment variable from here, where it is not mentioned...
(Single Shell): ...to here, where it is discussed along with
shell-file-name.
* lisp/dired.el (dired-insert-directory): Use shell-file-name
instead of explicit-shell-file-name when a shell is implicitly
requested.
* lisp/obsolete/terminal.el (explicit-shell-file-name):
* lisp/term.el (explicit-shell-file-name): Remove duplicate
defcustoms and load lisp/shell.el instead.  (Bug#40679)
* lisp/shell.el (explicit-shell-file-name): Clarify docstring.
(shell): Simplify.