doc: add limitations

[shahabganji]

Describe the limitation of Shadow Properties in the very first paragraph

[ajcvickers]

@shahabganji This change seems somewhat confusing to me since it implies that the CLR type has properties but they are not being set. Can you explain a bit more what limitation you are trying to convey?

[shahabganji]

@ajcvickers , I wanted to convey the difference between the following two queries:

1. With AsNoTracking()

 var authors = context.Authors.Select(x => x).AsNoTracking();

foreach (var author in authors)
{
    Console.WriteLine(
        $"The entity for AuthorId: {author.Id} " +
        $"was created at: {context.Entry(author).Property("CreatedOn").CurrentValue}");
}

2. Without AsNoTracking()

var authors2 = context.Authors.Select(x => x);
foreach (var author2 in authors2)
{
    Console.WriteLine(
        $"The entity for AuthorId: {author2.Id} " +
        $"was created at: {context.Entry(author2).Property("CreatedOn").CurrentValue}");
}

And the output will respectively be :

The entity for AuthorId: a2ab54b2-c72c-4644-8bf4-4fa48d3ee89d was created at: 01/01/0001 00:00:00 +00:00

The entity for AuthorId: a2ab54b2-c72c-4644-8bf4-4fa48d3ee89d was created at: 06/27/2020 16:31:47 +00:00

And for clarity, the CreatedOn is a Shadow Property and the Author class is like this:

public class Author
{
     public Guid Id { get; private set; }
     public string FirstName { get; private set; }
     public string LastName { get; private set; }
     public DateTime DateOfBirth { get; private set; }
}

[shahabganji]

I could also add a section with an example if you think that will provide a better understanding of the problem

[ajcvickers]

@shahabganji May add a sentence to Accessing shadow properties. Something like, "Shadow properties cannot be accessed after a no-tracking query since the entities returned are not tracked by the change tracker."

[shahabganji]

@ajcvickers, The changes applied.

[ajcvickers]

Thanks!