memory: update doc page

docs/20_Checks/check_memory.md

@@ -3,6 +3,7 @@
 ## Introduction
 
 **check_memory** Check memory usage incl. free, used and total memory.
+
 It reads /proc/meminfo and fetches the needed values.
 
 * MemTotal: Total usable ram (i.e. physical ram minus a few reserved bits and the kernel binary code)
@@ -16,6 +17,8 @@ It reads /proc/meminfo and fetches the needed values.
 * Slab: in-kernel data structures cache
 * SReclaimable: reclaimable parts of Slab, e.g., caches
 
+This plugin sends all values as performance data.
+
 ### Requirements
 
 * bc