Update document of security context

docs/security_context.md

 # Security Contexts
 
-A security context defines the operating system security settings (uid, gid, capabilities, SELinux role, etc..) applied to a container.
+A security context defines the operating system security settings (uid, gid, capabilities, SELinux role, etc..) applied to a container. See [security context design](design/security_context.md) for more details.
 
 
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