pgsql: doc: clarify how to find pg_type_d.h in the install tree

doc: clarify how to find pg_type_d.h in the install tree

Followup to patch 152ed04799.

Reported-by: Alvaro Herrera

Discussion: /messages/by-id/20201112202900.GA28098@alvherre.pgsql

Backpatch-through: 9.5

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5491d7829e7ed6726381cfbc90b2ce27c5016f8b

Modified Files
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doc/src/sgml/libpq.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)