MetaData#concreteIndices to throw exception with a single index argument if allowNoIndices == false

Fixed MetaData#concreteIndices to throw exception with a single index argument in case allowNoIndices == false and ignoreUnavailable == true.

The current behaviour is inconsistent as 2 or more non existing indices would cause an exception to be thrown:

```
GET /a,b/_search?allow_no_indices=false&ignore_unavailable=true

{
   "error": "IndexMissingException[[[a, b]] missing]",
   "status": 404
}
```

On the other hand 
```
GET /a/_search?allow_no_indices=false&ignore_unavailable=true
```

returns 0 results regardless of the `allow_no_indices` value.


Took also the change to improve the javadocs for `MetaData` class.