pgr_CreateTopology needs explaination of when things are updated

[robe2]

The warning on create topology http://docs.pgrouting.org/latest/en/pgr_createTopology.html#pgr-create-topology

Is confusing. It explains the more obvious but not the not so obvious like what does clean = true do to your edges and under what conditions does it not update the data. Suggestion:.

Move Warning up, to top, might be better as a notice or description

Should read:

`

The edge_table will may be affected

   If clean=true, source and target column values will be wiped out and reassigned.

  source and target columns will only be updated if they are not filled in already.

  Indexes will be created, if  they don't  exist, to speed up
  the process for the following columns:

            id
            the_geom
            source
            target

`

It should also note what happens with with vertices table if it's present and that it creates it if not present.