(checkdoc-this-string-valid-engine):

(checkdoc-this-string-valid-engine):
Improve messages for capitalization of arg names,
for flag variables, for t and nil, for imperatives.
Call them imperatives rather than infinitives sans "to".
(checkdoc-common-verbs-wrong-voice): Add "moves".
(checkdoc-message-text-engine): Improve messages for y-or-n-p.

(checkdoc-file-comments-engine):
Don't convert comments from 3-semicolon to 2-semicolon.
That is a matter of style.
(checkdoc-triple-semi-comment-check-flag): Variable deleted.
(checkdoc-comments): Don't bind that variable.