Provide guidance for missing value terms for collector.

[GoogleCodeExporter]

The HUH botanist list includes two terms for missing values for botanists (when 
used as collectors or other agents).  

[no data available]
http://purl.oclc.org/net/edu.harvard.huh/guid/uuid/edd03166-1ec7-45d7-9f7d-d5bd7
4e3e789

[illegible]  
http://purl.oclc.org/net/edu.harvard.huh/guid/uuid/eab40a35-3bd3-4699-a1bc-cb247
439e44b

The Macrofungal TCN has proposed the value: 
[collector unspecified] to use in cases where no collector was specified on the 
collector label. 

AppleCore should provide guidance on what values to use under what conditions.  

Original issue reported on code.google.com by mole@morris.net on 11 Jul 2013 at 3:15