manpage: TIFFSetTagExtender() cannot add tags to custom directories. Closes #557.
requested to merge Su_Laus/libtiff:fix_557_TagExtender_for_CustomDirectories_not_possible into master
Manpage update: TIFFSetTagExtender() cannot add tags to custom directories.
and some correction in addingtags.rst and typo.
Closes #557 (closed).