pgsql: docs: land height is "elevation", not "altitude"

docs: land height is "elevation", not "altitude"

See https://mapscaping.com/blogs/geo-candy/what-is-the-difference-between-elevation-relief-and-altitude
No patching of regression tests.

Reported-by: taf1@cornell.edu

Discussion: /messages/by-id/158506544539.679.2278386310645558048@wrigleys.postgresql.org

Backpatch-through: 9.5

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/92c12e46d5f1e25fc85608a6d6a19b8f5ea02600

Modified Files
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doc/src/sgml/advanced.sgml | 42 ++++++++++++++--------------
doc/src/sgml/ddl.sgml | 70 +++++++++++++++++++++++-----------------------
2 files changed, 56 insertions(+), 56 deletions(-)