DOC: add note on caching

src/doc/concepts.rst

@@ -173,3 +173,14 @@ Example:
 
 File Objects
 ============
+
+TODO
+
+Caching
++++++++
+
+The Crawler uses the cached library function ``cached_get_entity_by``. The
+cache is cleared automatically, when the Crawler does updates, but if you would
+run the same Python process indefinetely the Crawler would not see changes due
+to the Cache. Thus, please make sure to clear the cache if you create long
+running Python processes.