doc: clarify where to find pg_type_d.h (PG 11+) and pg_type.h
These files are in compiled directories and install directories.
Reported-by: e.indrupskaya@postgrespro.ru
Discussion: https://postgr.es/m/
160379609706.24746.
7506163279454026608@wrigleys.postgresql.org
Backpatch-through: 9.5