NO-JIRA: Add explanatory comment on PN_HANDLE to object.h
NO-JIRA: Add explanatory comment on PN_HANDLE to object.h

Several people have been confused by its usage, added an explanation.

astitcher/qpid-proton,apache/qpid-proton,alanconway/qpid-proton,ChugR/qpid-proton,ChugR/qpid-proton,gemmellr/qpid-proton,gemmellr/qpid-proton,alanconway/qpid-proton,ssorj/qpid-proton,gemmellr/qpid-proton,alanconway/qpid-proton,ssorj/qpid-proton,ssorj/qpid-proton,astitcher/qpid-proton,ChugR/qpid-proton,apache/qpid-proton,apache/qpid-proton,ChugR/qpid-proton,ChugR/qpid-proton,apache/qpid-proton,apache/qpid-proton,alanconway/qpid-proton,ssorj/qpid-proton,alanconway/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,alanconway/qpid-proton,kgiusti/qpid-proton,kgiusti/qpid-proton,apache/qpid-proton,astitcher/qpid-proton,astitcher/qpid-proton,kgiusti/qpid-proton,astitcher/qpid-proton,kgiusti/qpid-proton,gemmellr/qpid-proton-j,ssorj/qpid-proton,kgiusti/qpid-proton,gemmellr/qpid-proton,gemmellr/qpid-proton,gemmellr/qpid-proton-j,kgiusti/qpid-proton,ChugR/qpid-proton,ssorj/qpid-proton