Revise mmapfs requirements

* Move RPM/Debian note above for clarity.
* Add info on checking the setting before changing it
  I've noticed on my machine by default it has been set to 1048576

Author: Holek

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

@@ -1,11 +1,25 @@
 [[vm-max-map-count]]
 === Virtual memory
 
-Elasticsearch uses a <<mmapfs,`mmapfs`>> directory by
-default 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.
+Elasticsearch uses a <<mmapfs,`mmapfs`>> directory by default to store
+its indices. 
 
-On Linux, you can increase the limits by running the following command as
+Checking operating system limits on mmap counts might be
+advisable, otherwise running Elastic may result in out of memory exceptions.
+
+NOTE: On recent Linux distribution versions this limit should be
+sufficient. +
+The RPM and Debian packages will configure this setting automatically. In
+such case, no further configuration is required.
+
+To confirm the limits on Linux, you can run the following command:
+
+[source,sh]
+-------------------------------------
+sysctl vm.max_map_count
+-------------------------------------
+
+If needed, you can increase the limits by running the following command as
 `root`:
 
 [source,sh]
@@ -16,12 +30,10 @@ sysctl -w vm.max_map_count=262144
 To set this value permanently, update the `vm.max_map_count` setting in
 `/etc/sysctl.conf`. To verify after rebooting, run `sysctl vm.max_map_count`.
 
-The RPM and Debian packages will configure this setting automatically. No
-further configuration is required.
-
 You can find out the current mmap count of a running Elasticsearch process
 using the following command, where `$PID` is the process ID of the running
 Elasticsearch process:
+
 [source,sh]
 ----------------------------
 wc -l /proc/$PID/maps