(function-documentation): Make it work for the remaining cases

(function-documentation): Make it work for the remaining cases

* lisp/simple.el (function-documentation):
Use `internal-subr-documentation` and make it work also with symbols
and macros.

* src/doc.c (Fsubr_documentation): New function, extracted
from Fdocumentation.
(syms_of_doc): defsubr it.
(Fdocumentation): Don't handle subrs and module functions here.