Improve documentation of display-fill-column-indicator

Improve documentation of display-fill-column-indicator

* lisp/display-fill-column-indicator.el
(display-fill-column-indicator-mode): Mention the globalized
version in the doc string.

* doc/emacs/display.texi (Displaying Boundaries): Improve and
clarify the documentation of display-fill-column-indicator.
Suggest using the minor mode as the primary means for turning the
feature on.

* src/xdisp.c (syms_of_xdisp) <display-fill-column-indicator>
<display-fill-column-indicator-character>: Doc fix.  (Bug#44226)