SB-1475: add comment with explanation on what db migration does

Author: evsikov

strongbox-db-liquibase/src/main/resources/db/changelog/v1.0.0/v1.0.0.21__ArtifactEntry.xml

@@ -4,6 +4,24 @@
                    xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog
                         http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-3.2.xsd">
 
+    <!--
+        Problem:
+        Need to change class ArtifactEntry property 'created' type from 'date' into 'datetime'.
+        "ALTER PROPERTY ArtifactEntry.created TYPE datetime" fails to update property type.
+
+        Solution:
+        Apply sequence of two statements.
+        1. "DROP PROPERTY ArtifactEntry.created"
+        "Removes a property from the schema. Does not remove the property values in the records,
+        it just changes the schema information. Records continue to have the property values, if any."
+        For details see: https://orientdb.org/docs/3.1.x/sql/SQL-Drop-Property.html
+
+        2. "CREATE PROPERTY ArtifactEntry.created datetime"
+        "Creates a new property in the schema"
+        For details see: https://orientdb.org/docs/3.1.x/sql/SQL-Create-Property.html
+
+        Property values will retain previously existing data, but will have a new type 'datetime'
+-->
     <changeSet id="v1.0.0.21" author="strongbox-dev@carlspring.com">
 
         <o:dropProperty className="ArtifactEntry" propertyName="created"/>