Docs: multiCombo docs outdated? Add 'no'

[tordans]

https://github.com/ideditor/schema-builder#combodropdown-fields says

multiCombo - Dropdown field for adding yes values to multiple keys with the same prefix (a common multikey)
(e.g. recycling:* -> recycling:glass=yes, recycling:paper=yes, etc.)

But this was changed a while back, right? It is now yes>no>nil?

Ref openstreetmap/iD#5767 (comment)

Ref https://github.com/openstreetmap/id-tagging-schema/blob/main/data/fields/diet_multi.json