clarify search semantics

[kohlhase]

See https://trac.mathweb.org/MWS/ticket/1452, This is even more important if logical operators are admitted in the text slot. Then it becomes unclear what the query means at all. We shold discuss this and design the front-end input part accordingly (see #89 and extend it).

[Raduh]

Regarding MWS:
Right now, only one <expr> is allowed in the query. The rest are ignored.

Regarding ES:
Logical operators are not currently allowed in the text slot (they are treated as regular keywords).

[kohlhase]

Ah, interesting.
We should think about what we want to do here.