Add docstring to core_api_version (#204)

Follow-up to acquire-project/acquire-docs#85

Adding a docstring makes sure this object is visible in the
autogenerated API docs.

python/acquire/acquire.pyi

@@ -1404,4 +1404,6 @@ class VoltageRange:
         """Returns a dictionary of the `VoltageRange` object's attributes."""
         ...
 
-def core_api_version() -> str: ...
+def core_api_version() -> str:
+    """Returns the version string for the core API."""
+    ...