BUG #6556: earthdistance module has lacking documentation

The following bug has been logged on the website:

Bug reference:      6556
Logged by:          Kasper Sandberg
Email address:      kontakt@sandberg-consult.dk
PostgreSQL version: 9.1.3
Operating system:   -
Description:=20=20=20=20=20=20=20=20

looking at http://www.postgresql.org/docs/9.1/static/earthdistance.html

it looks very scarce and missing, and conflicting, some things operate based
on miles, others meters. it also doesnt specify the units used for
earth_distance() which seems to be meters.