[Checkers] Note regarding Custom Checker

[Mahmood Ali]

Hi guys,

I will stop my email stream after this one (as they may be  
distracting).  I just wanted to point out that the Custom Checker now  
can express incompatible types, like Encrypted and Plaintext.  Namely,  
the following relationship

Encrypted <: null
Plaintext <: null
Encrypted !<: Plaintext
Plaintext !<: Entrypted

and the custom checker issues an error for
	@PlainText str = myEncryptedData;

AnnotationRelations treats any two qualifiers in the hierarchy without  
subtype relationship as 'in conflict' relationship and conduce an  
error for any assignment from one to the other.

However, Custom Checker does not issue an error if both annotations  
are present on one type.

Regards,
Mahmood