No more using 'hybrid mmapfs / niofs' (#25944)

It looks a bit ambiguous here.

ElasticSearch no more using 'hybrid mmapfs / niofs' which chooses filesystem based on the file. It is any one of the mmapfs, niofs or simplefs depending on the operating system.
As quoted here https://www.elastic.co/guide/en/elasticsearch/reference/5.5/index-modules-store.html

Thanks,
Pulkit Agrawal

docs/reference/setup/sysconfig/virtual-memory.asciidoc

@@ -1,8 +1,8 @@
 [[vm-max-map-count]]
 === Virtual memory
 
-Elasticsearch uses a <<default_fs,`hybrid mmapfs / niofs`>> directory by
-default to store its indices.  The default operating system limits on mmap
+Elasticsearch uses a <<default_fs,`mmapfs`>> directory by
+default for 64bit systems to store its indices.  The default operating system limits on mmap
 counts is likely to be too low, which may result in out of memory exceptions.
 
 On Linux, you can increase the limits by running the following command as