Some preset fields like the access-restriction field manage
multiple tags at once (access=*, foot=*, bicycle=*, …). Then
iD wasn't able to show inline help when clicking the ⓘ button.
This introduces a new parameter for these kind of fields:
`referenceKey` determines the tag key which should be used to
search taginfo documentation for.
Fixes#2222.
* It shouldn't appear as a search result for simple areas.
* Changing to another area preset shouldn't remove the
type=multipolygon tag.
* It shouldn't be preferred to the Building preset, which
matches with score 0.5.
Fixes#1546.
