[DOCS] Added pages explaining lucene query parser syntax and regular expression syntax
Rejecting invalid fields inside of completion field

Only valid fields (input, output, weight and payload) are now allowed inside a completion field.
This makes sure that typos like "inputs" are caught on indexing.
Improve exception handling for river startup

Due to a change in 0.90 some exceptions may show up when starting a river,
even though this is not a problem as the shard has not been initialized
yet and a retry is scheduled.
Remove @Required annotation

The @Required annotation is used for documentation purposes only anyway.