[ADF-1586] Added docs for services (#2619)

* [ADF-1586] Added docs for services

* [ADF-1586] Removed model definitions from docs

docs/user-process.model.md

@@ -0,0 +1,21 @@
+# User Process model
+
+Represents a Process Services user.
+
+## Details
+
+This class contains basic information about a Process Services user and
+is used throughout ADF to identify and list users (eg, to assign them to
+a task or to list them in search results).
+
+Note that the Typescript class for this model is in active development;
+it is likely to change and so its properties are not listed here. For the
+latest version of the class, see the
+[source file](https://github.com/Alfresco/alfresco-ng2-components/blob/development/ng2-components/ng2-alfresco-core/src/models/user-process.model.ts).
+
+<!-- Don't edit the See also section. Edit seeAlsoGraph.json and run config/generateSeeAlso.js -->
+<!-- seealso start -->
+## See also
+
+- [People process service](people-process.service.md)
+<!-- seealso end -->