docs: move abstract classes topic to handbook

(cherry picked from commit 23d1e06)

Author: james

docs/language/learn-ql/advanced/abstract-classes.rst

@@ -10,7 +10,6 @@ Advanced QL
 
 Topics on advanced uses of QL. These topics assume that you are familiar with QL and the basics of query writing.
 
--  :doc:`Semantics of abstract classes <abstract-classes>`
 -  :doc:`Choosing appropriate ways to constrain types <constraining-types>`
 -  :doc:`Determining the most specific types of a variable <determining-specific-types-variables>`
 -  :doc:`Monotonic aggregates in QL <monotonic-aggregates>`

docs/language/learn-ql/advanced/advanced-ql.rst

@@ -208,7 +208,9 @@ by declaring them in the ``from`` part.
 You can also annotate predicates and fields. See the list of :ref:`annotations <annotations-overview>`
 that are available.
 
-Kinds of classes
+.. _concrete-classes:
+
+Concrete classes
 ================
 
 The classes in the above examples are all **concrete** classes. They are defined by 
@@ -218,6 +220,9 @@ values in the intersection of the base types that also satisfy the
 
 .. _abstract-classes:
 
+Abstract classes
+================
+
 A class :ref:`annotated <abstract>` with ``abstract``, known as an **abstract** class, is also a restriction of 
 the values in a larger type. However, an abstract class is defined as the union of its 
 subclasses. In particular, for a value to be in an abstract class, it must satisfy the 
@@ -247,6 +252,13 @@ The abstract class ``SqlExpr`` refers to all of those different expressions. If
 support for another database system later on, you can simply add a new subclass to ``SqlExpr``;
 there is no need to update the queries that rely on it.
 
+.. pull-quote:: Important
+
+
+   You must take care when you add a new subclass to an existing abstract class. Adding a subclass
+   is not an isolated change, it also extends the abstract class since that is a union of its
+   subclasses. 
+
 .. _overriding-member-predicates:
 
 Overriding member predicates