Modify guidance on negative "symerror" now that the code supports it
Modify guidance on negative "symerror" now that the code supports it

Signed-off-by: Graeme Watt <70d9459688464dad39678b2e525a04541a05c5e3@durham.ac.uk>

HEPData/hepdata-submission,HEPData/hepdata-submission,HEPData/hepdata-submission