Merge pull request #5955 from planetscale/ds-region-experiment-doc

RegionExperimental: improve doc

go/vt/vtgate/vindexes/region_experimental.go

@@ -33,9 +33,10 @@ func init() {
 	Register("region_experimental", NewRegionExperimental)
 }
 
-// RegionExperimental defines a vindex that uses a lookup table.
-// The table is expected to define the id column as unique. It's
-// Unique and a Lookup.
+// RegionExperimental is a multi-column unique vindex. The first column is prefixed
+// to the hash of the second column to produce the keyspace id.
+// RegionExperimental can be used for geo-partitioning because the first column can denote a region,
+// and its value will dictate the shard for that region.
 type RegionExperimental struct {
 	name        string
 	regionBytes int