Improve viewing plain text contents of document

* doc/emacs/misc.texi (Document View): Adapt to latest doc-view changes wrt
viewing the document's plain text contents.

* lisp/doc-view.el (doc-view-open-text): View the document's plain text
in the current buffer instead of a new one.
(doc-view-toggle-display): Handle the case where the current
buffer contains the plain text contents of the document.
(doc-view-initiate-display): Don't switch to fallback mode if the
user wants to view the doc's plain text.
(doc-view-set-doc-type): Use assoc-string instead of
assoc-ignore-case.