Fix assertions in testEquivalence and beef up the test
Catch AlreadClosedException in RobinSearcher#release()

When we relocate a shard we might still have pending SearchContext
instances hanging around that will be used in "in-flight" searches
on the already relocated shard. This is a valid operation but if
we have already closed the underlying directory which happens during
cleanup concurrently the close call on the IndexReader can trigger
an AlreadyClosedException when the NRT reader tries to cleanup files
via the IndexWriter.

Closes #4273
[DOCS] Documented rest.action.multi.allow_explicit_index
[DOCS] Fixed a bad link