Confusing doc string

[Jason Rumney]

The doc string for compilation-search-path has the confusing sentence 
"Elements should be directory names, not file names of directories."
I don't know what that is supposed to mean, can someone clarify it?

   compilation-search-path is a variable defined in `compile.el'.
   Its value is (nil)

   Documentation:
   *List of directories to search for source files named in error messages.
   Elements should be directory names, not file names of directories.
   nil as an element means to try the default directory.