Improve doc.

Change-Id: I0b25fd1c645a47084fb7fda1bbb7d41a2d10dcbb

doc/source/protocol/descprop.rst

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+Descriptive properties
+======================
+
+Mandatory descriptive properties
+--------------------------------
+
+parameter-properties
+++++++++++++++++++++
+
+* name (implicit)
+* datatype
+* readonly (bool)
+
+module-properties
++++++++++++++++++
+
+* interface_class [list_of_strings] (MAY be empty)
+
+SEC-Node-properties
++++++++++++++++++++
+
+* no mandatory properties
+
+Optional descriptive properties
+-------------------------------
+
+parameter-properties
+++++++++++++++++++++
+
+* unit (string), SHOULD be given if meaningful (if not given: unitless) (empty string: unit is one)
+* description (string), SHOULD be given
+* visibility
+* group (identifier) (MUST start with an uppercase letter) (if empty string: treat as not specified)
+
+module-properties
++++++++++++++++++
+
+* description (string), SHOULD be given
+* visibility
+* group (identifier) (MUST start with an uppercase letter) (if empty string: treat as not specified)
+* meaning ???
+* importance ???
+
+SEC-Node-properties
++++++++++++++++++++
+
+* description (string), SHOULD be given
+