clarify mkpath docstring (#43987)

Close #43964

base/file.jl

@@ -197,7 +197,8 @@ end
 Create all intermediate directories in the `path` as required. Directories are created with
 the permissions `mode` which defaults to `0o777` and is modified by the current file
 creation mask. Unlike [`mkdir`](@ref), `mkpath` does not error if `path` (or parts of it)
-already exists. Return `path`.
+already exists. However, an error will be thrown if `path` (or parts of it) points to an
+existing file. Return `path`.
 
 If `path` includes a filename you will probably want to use `mkpath(dirname(path))` to
 avoid creating a directory using the filename.