(declare-function): Add optional fourth argument and document it.

lisp/emacs-lisp/byte-run.el

@@ -103,7 +103,7 @@ The return value of this function is not used."
      (eval-and-compile
        (put ',name 'byte-optimizer 'byte-compile-inline-expand))))
 
-(defmacro declare-function (fn file &optional arglist)
+(defmacro declare-function (fn file &optional arglist fileonly)
   "Tell the byte-compiler that function FN is defined, in FILE.
 Optional ARGLIST is the argument list used by the function.  The
 FILE argument is not used by the byte-compiler, but by the
@@ -120,6 +120,15 @@ declaration.  A FILE with an \"ext:\" prefix is an external file.
 `check-declare' will check such files if they are found, and skip
 them without error if they are not.
 
+FILEONLY non-nil means that `check-declare' will only check that
+FILE exists, not that it defines FN.  This is intended for
+function-definitions that `check-declare' does not recognize, e.g.
+`defstruct'.
+
+To specify a value for FILEONLY without passing an argument list,
+set ARGLIST to `t'.  This is necessary because `nil' means an
+empty argument list, rather than an unspecified one.
+
 Note that for the purposes of `check-declare', this statement
 must be the first non-whitespace on a line, and everything up to
 the end of FILE must be all on the same line.  For example: