Moved sections from user guide to separate pages

docs/guide/document-instances.rst

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+===================
+Documents instances
+===================
+To create a new document object, create an instance of the relevant document
+class, providing values for its fields as its constructor keyword arguments.
+You may provide values for any of the fields on the document::
+    
+    >>> page = Page(title="Test Page")
+    >>> page.title
+    'Test Page'
+
+You may also assign values to the document's fields using standard object 
+attribute syntax::
+
+    >>> page.title = "Example Page"
+    >>> page.title
+    'Example Page'
+
+Saving and deleting documents
+-----------------------------
+To save the document to the database, call the
+:meth:`~mongoengine.Document.save` method. If the document does not exist in
+the database, it will be created. If it does already exist, it will be
+updated.
+
+To delete a document, call the :meth:`~mongoengine.Document.delete` method.
+Note that this will only work if the document exists in the database and has a
+valide :attr:`id`.
+
+.. seealso::
+    :ref:`guide-atomic-updates`
+
+Document IDs
+------------
+Each document in the database has a unique id. This may be accessed through the
+:attr:`id` attribute on :class:`~mongoengine.Document` objects. Usually, the id
+will be generated automatically by the database server when the object is save,
+meaning that you may only access the :attr:`id` field once a document has been
+saved::
+
+    >>> page = Page(title="Test Page")
+    >>> page.id
+    >>> page.save()
+    >>> page.id
+    ObjectId('123456789abcdef000000000')
+
+Alternatively, you may define one of your own fields to be the document's
+"primary key" by providing ``primary_key=True`` as a keyword argument to a
+field's constructor. Under the hood, MongoEngine will use this field as the
+:attr:`id`; in fact :attr:`id` is actually aliased to your primary key field so
+you may still use :attr:`id` to access the primary key if you want::
+
+    >>> class User(Document):
+    ...     email = StringField(primary_key=True)
+    ...     name = StringField()
+    ...
+    >>> bob = User(email='bob@example.com', name='Bob')
+    >>> bob.save()
+    >>> bob.id == bob.email == 'bob@example.com'
+    True
+
+.. note::
+   If you define your own primary key field, the field implicitly becomes
+   required, so a :class:`ValidationError` will be thrown if you don't provide
+   it.