lib.fileset: Minor contributor doc adjustments

- `_internalBase` (path):

  Any files outside of this path cannot influence the set of files.

  This is always a directory.

  This is always a directory and should be as long as possible.

  This is used by `lib.fileset.toSource` to check that all files are under the `root` argument

- `_internalBaseRoot` (path):

  The filesystem root of `_internalBase`, same as `(lib.path.splitRoot _internalBase).root`.

### Empty directories

File sets can only represent a _set_ of local files, directories on their own are not representable.

File sets can only represent a _set_ of local files.

Directories on their own are not representable.

Arguments:

- (+) There does not seem to be a sensible set of combinators when directories can be represented on their own.