Document limitations of interrupting a query

[danielmewes]

http://rethinkdb.com/docs/system-jobs/ briefly mentions terminating queries:

The jobs table [...] allows you to kill query jobs by deleting them from the table.

I think we should elaborate on that a little bit and note that not all queries will terminate immediately. Specifically, many queries will only get terminated the next time they emit a result, but not earlier.
A typical example is r.table(...).count(), which will never get terminated before its done, since it only emits a single result at the end.
See rethinkdb/rethinkdb#5197 for another example.