DOC: Started documentation of internal tables.

doc/multipurpose_subdomains.md

+# Multipurpose subdomains #
+Multipurpose subdomains are generically used to work with composite
+properties. Currently implemented examples are:
+- Properties with units (if multiple Properties exist)
+- Lists
+- Name overrides
+
+## Example ##
+Let's have a look at this *Record* (simplified XML):
+
+```xml
+<R1>
+    <P1 name="Comment">Hello World</P1>
+    <P2 name="voltage" unit="V">
+        23
+    </P2>
+    <P3 comment="list of something">
+        V1, V2, V3, V4, ...
+    </P3>
+</R1>
+```
+