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