Facilitate search for terms in selected works only

[dvglc]

• short-term (v1.6): "How To" in search help for selection through the query form

• long-term (v2.0+): perhaps offer selection options via GUI (e.g., forms, selectable boxes, ...) - still to be discussed.

[dvglc]

With regards to the "short term (v1.5)" goal (which I postpone to v1.6 for now):

Our search docs say that the "@" field operator (which to my understanding would be necessary to search for single works) isn't allowed in query fields.

Also, I now think that a search using the field operator (if it was available) like so: "@work_id W0004", demands too much from the user, who needs to know about the work id scheme in our digital edition, then search the right id, then enter it with correct syntax into the query field... I suggest that we rather focus on the "long-term", v2.0 goal, as stated above.