Clarify the definition of "local"

Most people reading this sentence probably don't have PEP 440 memorized.

Author: maresb

docs/config.md

@@ -15,7 +15,7 @@ Callables or other Python objects have to be passed in `setup.py` (via the `use_
   See [Version number construction](extending.md#setuptools_scmversion_scheme) for predefined implementations.
 
 `local_scheme : str | Callable[[ScmVersion], str]`
-: Configures how the local component of the version is constructed
+: Configures how the local component of the version (the optional part after the `+`) is constructed;
   either an entrypoint name or a callable.