update documentation on organismQuantity interpretation

[CecSve]

This is how we currently interpret organismQuantity - note we only interpret numbers, although the DwC QRG says both numbers and enumerations are allowed:

Definition: A number or enumeration value for the quantity of dwc:Organisms.
Comments: A dwc:organismQuantity must have a corresponding dwc:organismQuantityType.
Examples:

• 27 (organismQuantity) with individuals (organismQuantityType)
• 12.5 (organismQuantity) with % biomass (organismQuantityType)
• r (organismQuantity) with Braun-Blanquet Scale (organismQuantityType)
• many (organismQuantity) with individuals (organismQuantityType)

It should be updated in:

https://techdocs.gbif.org/en/data-use/download-formats#dwca-interpreted @MattBlissett
https://techdocs.gbif.org/en/openapi/v1/occurrence#/Searching%20occurrences/searchOccurrence @MattBlissett
https://techdocs.gbif.org/en/data-publishing/data-quality-recommendations#organismQuantity @CecSve

[CecSve]

We may want to explore a controlled vocabulary: gbif/vocabulary#181.