Allow splitting strings in Eshell expansions with "plain" strings

Allow splitting strings in Eshell expansions with "plain" strings

Since '$var[hello 0]' doesn't make sense when 'var' is a string, the
previous restriction was unnecessary.

* lisp/eshell/esh-var.el (Commentary): Update documentation.
(eshell-apply-indices): Allow "plain" strings to split strings.

* test/lisp/eshell/esh-var-test.el
(esh-var-test/interp-var-string-split-indices)
(esh-var-test/quoted-interp-var-string-split-indices): Update tests.

* doc/misc/eshell.texi (Dollars expansion): Update documentation.