Added note in docs for cluster.routing.allocation filters.
Added note in docs for cluster.routing.allocation filters.

.. to point out that applying such filters leads to deallocation of
nodes where filters do not match.

EvilMcJerkface/crate,EvilMcJerkface/crate,crate/crate,crate/crate,EvilMcJerkface/crate,crate/crate