relative to the Emacs \"src/\" directory. Lisp files are
searched for using `locate-library', and if that fails they are
expanded relative to the location of the file containing the
-declaration.
+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.
Note that for the purposes of `check-declare', this statement
must be the first non-whitespace on a line, and everything up to