In function definitions, doc strings, `declare` and `interactive` forms are each optional but must appear in that order and duplications are not permitted. Many violations against these rules go undetected.

The attached patch adds checks to `defun` and `defmacro`. Violations result in compilation warnings but it could be changed into hard errors if desired. It also accepts `(:documentation EXPR)` as alternative to a doc string literal, which currently doesn't work i all cases.

Applying it reveals a handful of mistakes in the Emacs source tree, and in some external packages.