Document Active Anti-Entropy incompatibility with Bitcask Expiry

[jcapricebasho]

Our recommendation against using Active Anti-Entropy with Bitcask Expiry needs to be documented, likely here: http://docs.basho.com/riak/latest/theory/concepts/aae/

The usage of Active Anti-Entropy with Bitcask Expiry performs superfluous work, and even has the potential to resurrect data that Bitcask has recently expired.

[dmitrizagidulin]

To put it more strongly, Engineering straight up says: usage of Bitcask+Expiry with AAE is unsupported.

More importantly, this has implications in the "Automatic expiration and Riak Search" section of http://docs.basho.com/riak/latest/ops/advanced/backends/bitcask/ --
Currently, it says that when using Search with Automatic Expiration on Bitcask, make sure to turn on AAE (to prevent orphaned index entries). But usage of AAE + Expiration is unsupported.
Therefore, don't use Search with Expiration.