Document allowed index type for SQL

[TarantoolBot]

Before the change, Tarantool query planner segfaulted on
try of using non-tree index. It is blocked now w/ appropriate
error message. Need to document the behaviour.
It should be noted, that this restriction might be relaxed in future.

Closes #4659
Requested by @kyukhin in tarantool/tarantool@e7a70be.

[pgulutzan]

@kyukhin: In February I removed the hash-index example after it became clear from issue tarantool/tarantool#4659 comments that their non-support was intentional. The SQL section of the manual currently says: "Fields in NoSQL spaces can be accessed with SQL if and only if they are scalar and are defined in format clauses. Indexes of NoSQL spaces will be used with SQL if and only if they are TREE indexes." Seeing this, would you agree that no change is needed?

[NickVolynkin]

@kyukhin please reply to the previous message.

[kyukhin]

@kyukhin: In February I removed the hash-index example after it became clear from issue tarantool/tarantool#4659 comments that their non-support was intentional. The SQL section of the manual currently says: "Fields in NoSQL spaces can be accessed with SQL if and only if they are scalar and are defined in format clauses. Indexes of NoSQL spaces will be used with SQL if and only if they are TREE indexes." Seeing this, would you agree that no change is needed?

I guess so.