Improve custom migration documentation.
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<hr/>
</p>
+<p>
+A common pitfall when using custom migrations is the deletion of features. When a custom migration is attached to a change or
+operation which results in the deletion of an EStructuralFeature, the values for deleted feature are only available in the
+#migrateBefore method of the custom migration. In #migrateAfter the metamodel change has already been performed, so trying to
+read old values from the model won't work. In this case implementors need to store the state in #migrateBefore (e.g. in a Map)
+and then retrieve them again in #migrateAfter.
+</p>
+
<h2 id="migrator">Migrator</h2>
<p>
