Update cassandra.yaml comments to reflect memory_allocator deprecation, remove in 3.0

Looks like in 2.2+ changing the memory_allocator field in cassandra.yaml has no effect:

https://github.com/apache/cassandra/commit/0d2ec11c7e0abfb84d872289af6d3ac386cf381f#diff-b66584c9ce7b64019b5db5a531deeda1R207

The instructions in comments on how to use jemalloc haven't been updated to reflect this change. Robert Stupp, is that an accurate assessment?

[~iamaleksey] How do we want to handle the deprecation more generally? Warn on 2.2, remove in 3.0?

EDIT: I described this in a way that isn't very clear to those unfamiliar with Robert's change. To be clear: You can still use JEMAlloc with Cassandra. The memory_allocator option was deprecated in favor of automatically detecting if JEMAlloc is installed, and, if so, using it.