Add sysctl to glossary (#12607)

* Add sysctl to glossary

* Make it clear that sysctl is for Unix-like OSs

content/en/docs/reference/glossary/sysctl.md

@@ -0,0 +1,23 @@
+---
+title: sysctl
+id: sysctl
+date: 2019-02-12
+full_link: /docs/tasks/administer-cluster/sysctl-cluster/
+short_description: >
+  An interface for getting and setting Unix kernel parameters
+
+aka:
+tags:
+- tool
+---
+ `sysctl` is a semi-standardized interface for reading or changing the
+ attributes of the running Unix kernel.
+
+<!--more-->
+
+On Unix-like systems, `sysctl` is both the name of the tool that administrators
+use to view and modify these settings, and also the system call that the tool
+uses.
+
+{{< glossary_tooltip text="Container" term_id="container" >}} runtimes and
+network plugins may rely on `sysctl` values being set a certain way.