Update saving-your-data.rst

The wording of the description for create() was very vague, misleading, complicated to understand.  The 2.3 API description of create() is much better, but still a bit lacking.  After tracing through the code to discover just what create() actually does and when and then doing some tests of my own, I've written some changes to the description which more accurately and effectively communicate what create() actually does.

1.) The previous description strongly hinted that create() actually had no effect on fields that "have already been set" prior to calling create() and only upon fields which "have already been set", which is not the case. Any data that is set prior to calling create() is actually overwritten completely by create().  I've updated the description to accurately describe the true effect upon Model::$data.

2.) I've added the fact that null may also be passed as the $data param, not just false.  This was in the API but not yet added here in the docs.

3.) I've added clarity to what happens when there are no database defaults AND no parameters passed for $data. In this case, Model::$data is set to an empty array.

4.) I've added clarity to what happens when false or null are passed for $data, regardless of there being any database defaults. In this case, Model::$data is set to an empty array.