Adjust documentation of exposed API

I stumbled across this being a potential point of confusion in the
Exifread mapping repository's PR 21.

References:
* casework/CASE-Implementation-PyPI-Exifread#21

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

case_utils/local_uuid.py

@@ -13,10 +13,14 @@
 
 """
 This library is a wrapper for uuid, provided to generate repeatable UUIDs if requested.
+
+The function local_uuid() should be used in code where a user could be expected to opt in to non-random UUIDs.
 """
 
 __version__ = "0.3.2"
 
+__all__ = ["configure", "local_uuid"]
+
 import logging
 import os
 import pathlib
@@ -34,13 +38,13 @@
 
 def configure() -> None:
     """
-    This function is part of setting up demo_uuid() to generate non-random UUIDs.  See demo_uuid() documentation for further setup notes.
+    This function is part of setting up _demo_uuid() to generate non-random UUIDs.  See _demo_uuid() documentation for further setup notes.
     """
     global DEMO_UUID_BASE
 
     if os.getenv("DEMO_UUID_REQUESTING_NONRANDOM") == "NONRANDOM_REQUESTED":
         warnings.warn(
-            "Environment variable DEMO_UUID_REQUESTING_NONRANDOM is deprecated.  See case_utils.local_uuid.demo_uuid for usage notes on its replacement, CASE_DEMO_NONRANDOM_UUID_BASE.  Proceeding with random UUIDs.",
+            "Environment variable DEMO_UUID_REQUESTING_NONRANDOM is deprecated.  See case_utils.local_uuid._demo_uuid for usage notes on its replacement, CASE_DEMO_NONRANDOM_UUID_BASE.  Proceeding with random UUIDs.",
             FutureWarning,
         )
         return
@@ -109,10 +113,12 @@ def configure() -> None:
     DEMO_UUID_BASE = "/".join(demo_uuid_base_parts)
 
 
-def demo_uuid() -> str:
+def _demo_uuid() -> str:
     """
     This function generates a repeatable UUID, drawing on non-varying elements of the environment and process call for entropy.
 
+    This function is not intended to be called outside of this module.  Instead, local_uuid() should be called.
+
     WARNING: This function was developed for use ONLY for reducing (but not eliminating) version-control edits to identifiers when generating sample data.  It creates UUIDs that are decidedly NOT random, and should remain consistent on repeated calls to the importing script.
 
     To prevent accidental non-random UUID usage, two setup steps need to be done before calling this function: