DOC: added more documentation for the high level API

src/doc/high_level_api.org

@@ -91,3 +91,31 @@ print(str(obj))
 
 * Concepts
 
+As described in the section [[Quickstart]] the two functions "convert_to_python_object" and "convert_to_entity" convert
+entities beetween the high level and the standard representation.
+
+The high level entities are represented using the following classes from the module caosdb.high_level_api:
+- CaosDBPythonEntity: Base class of the following entity classes.
+- CaosDBPythonRecord
+- CaosDBPythonRecordType
+- CaosDBPythonProperty
+- CaosDBPythonMultiProperty: **WARNING** Not implemented yet.
+- CaosDBPythonFile: Used for file entities and provides an additional "download" function for being able to directly retrieve files from CaosDB.
+
+In addition, there are the following helper structures which are realized as Python data classes:
+- CaosDBPropertyMetaData: For storing meta data about properties.
+- CaosDBPythonUnresolved: The base class of unresolved "things".
+- CaosDBPythonUnresolvedParent: Parents of entities are stored as unresolved parents by default, storing an id or a name of a parent (or both).
+- CaosDBPythonUnresolvedReference: An unresolved reference is a reference property with an id which has not (yet) been resolved to an Entity.
+
+The function "resolve_references" can be used to recursively replace CaosDBPythonUnresolvedReferences into members of type CaosDBPythonRecords
+or CaosDBPythonFile.
+
+Each property stored in a CaosDB record corresponds to:
+- a member attribute of CaosDBPythonRecord **and**
+- an entry in a dict called "metadata" storing a CaosDBPropertyMetadata object with the following information about proeprties:
+  - unit
+  - datatype
+  - description
+  - id
+  - importance