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: https://postgr.es/m/158506544539.679.2278386310645558048@wrigleys.postgresql.org

Backpatch-through: 9.5