Add some notes about String::isNull() and String::null.
Add some notes about String::isNull() and String::null.

TsudaKageyu/taglib,black78/taglib,taglib/taglib,TsudaKageyu/taglib,taglib/taglib,Distrotech/taglib,Distrotech/taglib,black78/taglib,taglib/taglib,Distrotech/taglib,TsudaKageyu/taglib,black78/taglib