Is there any way to disable just this one warning? Two spaces seems like overkill but maybe that is the lisp standard?
checkdoc-current-buffer to implement the
emacs-lisp-checkdoc checker which (in general) takes
sentence-end-double-space into account. However, the checker itself is implemented by calling an emacs subprocess and only passing along the variables whitelisted in
flycheck-emacs-lisp-checkdoc-variables, which doesn't include
sentence-end-double-space. You can (but since it's a defconst shouldn't) add
flycheck-emacs-lisp-checkdoc-variables to pass it along as well:
(setq-local flycheck-emacs-lisp-checkdoc-variables (cons 'sentence-end-double-space flycheck-emacs-lisp-checkdoc-variables))