+doc, 'sample' to 'population', +link and sorted

I'm only changing the documentation here, not SQL!

The inline documentation is used to generate http://www.ensembl.org/info/docs/api/variation/variation_schema.html#allele

The documentation for a couple of columns has outdated references to 'samples' where the table now uses population_id.

I added a link to the submitter_handle table, so all FKs are now linked under the 'see also' list.

I re-ordered the columns to match what's in the SQL.

I re-ordered the 'see also' list to match the order of the columns.