Various minor checkdoc tweaks

Various minor checkdoc tweaks

* lisp/emacs-lisp/checkdoc.el (checkdoc-symbol-words): Add more
"good" words that are used a lot in practice, and where using them
doesn't really hurt the quality of the documentation.
(checkdoc-proper-noun-list): Add "dired", remove "ispell"; the
latter should not always be capitalized.
(checkdoc-common-verbs-wrong-voice): Add some more common words.
Don't check for "matches" as it leads to too many false positives and
almost no fixes in practice.
(checkdoc-this-string-valid-engine): Clarify comment.
(checkdoc-in-abbreviation-p): Ignore some less common or non-standard
abbreviations.