docs: add note about field comparison

See #574

Author: msiemens

docs/getting-started.rst

@@ -112,5 +112,12 @@ Before we dive deeper, let's recapitulate the basics:
 |                               | ``== 2`` (also possible: ``!=``, ``>``, ``>=``, ``<``, ``<=``)|
 +-------------------------------+---------------------------------------------------------------+
 
+.. note::
+
+    Query comparisons only support literal values on the right-hand side.
+    Field-to-field comparisons like ``Query().a == Query().b`` are not
+    supported. Use a callable predicate like
+    ``db.search(lambda doc: doc.get('a') == doc.get('b'))`` for custom logic.
+
 .. References
 .. _GitHub: http://github.com/msiemens/tinydb/

docs/usage.rst

@@ -203,6 +203,11 @@ queries:
 
     You can compose queries dynamically by using the no-op query ``Query().noop()``.
 
+    Comparisons only support literal values on the right-hand side. Field-to-field
+    comparisons like ``Query().a == Query().b`` are not supported. Use a callable
+    predicate like ``db.search(lambda doc: doc.get('a') == doc.get('b'))`` for
+    custom logic.
+
 Recap
 .....