Split ORM and WTF fields documentation

Author: insspb

docs/db_model.md

@@ -0,0 +1,47 @@
+# Database model and fields definition
+
+```{important}
+Flask-Mongoengine does not adjust database level behaviour of [mongoengine] fields
+definition, except [keyword only definition] requirement. Everything other on
+database level match [mongoengine] project. All parent methods, arguments (as
+keyword arguments) and keyword arguments are supported.
+
+If you are not intend to use WTForms integration, you are free to use fields classes
+from parent [mongoengine] project; this should not break anything.
+```
+
+## Supported fields
+
+Flask-Mongoengine support all **database** fields definition. Even if there will be some
+new field type created in parent [mongoengine] project, we will silently bypass
+field definition to it, if we do not declare rules on our side.
+
+```{note}
+Version **2.0.0** Flask-Mongoengine update support [mongoengine] fields, based on
+version **mongoengine==0.21**. Any new fields bypassed without modification.
+```
+
+## Keyword only definition
+
+```{eval-rst}
+.. versionchanged:: 2.0.0
+```
+
+Database model definition rules and Flask-WTF/WTForms [integration] was seriously
+updated in version **2.0.0**. Unfortunately, these changes implemented without any
+deprecation stages.
+
+Before version **2.0.0** Flask-Mongoengine integration allowed to pass fields
+parameters as arguments. To exclude any side effects or keyword parameters
+duplication/conflicts, since version **2.0.0** all fields require keyword
+only setup.
+
+Such approach removes number of issues and questions, when users frequently used
+Flask-WTF/WTForms definition rules by mistake, or just missed that some arguments
+was passed to keyword places silently creating unexpected side effects. You can
+check issue [#379] as example of one of such cases.
+
+[mongoengine]: https://docs.mongoengine.org/
+[#379]: https://github.com/MongoEngine/flask-mongoengine/issues/379
+[integration]: forms
+[keyword only definition]: #keyword-only-definition