Document usage of change tracker clearing before calling test

[ArwynFr]

Add these informations to the usage documentation:

The library uses a singleton instance of the DbContext. Any calls to the DbContext during the arrange phase (either directly or through the HttpClient) might clogger the Change Tracker with existing entities. These tracked entities may cause attaching objects to fail within the context of the test, whereas it would have worked with a clean scoped DbContext instance. Is such case, the test writer should call Database.ChangeTracker.Clean() between arrange and acting calls.