geo_ops.c: Clarify comments and function arguments
geo_ops.c: Clarify comments and function arguments

These functions were not crystal clear about what their respective APIs
are.  Make an effort to improve that.

Emre's patch was correct AFAICT, but I (Álvaro) felt the need to improve
a few comments a bit more.  Any resulting errors are my own.

Per complaint from Coverity, Ning Yu, and Tom Lane.

Author: Emre Hasegeli, Álvaro Herrera
Reviewed-by: Tomas Vondra, Álvaro Herrera
Discussion: eac07fe93e5687f80c810ed691e391154e6e5d27@sss.pgh.pa.us

xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb