Added documentation for useringroup policy function

Ticket: ENT-12721
Signed-off-by: Victor Moene <[email protected]>

Author: victormlg

reference/functions/useringroup.markdown

@@ -0,0 +1,21 @@
+---
+layout: default
+title: useringroup
+published: true
+---
+
+[%CFEngine_function_prototype(user)%]
+
+**Description:** Return whether a `user` is a member of a given `group`
+
+Looks up the group `"group_name"` in the group database located in /etc/group and checks whether `"user_name"` is one of its members.
+
+[%CFEngine_function_attributes(user_name, group_name)%]
+
+**Example:**
+
+[%CFEngine_include_snippet(useringroup.cf, #\+begin_src cfengine3, .*end_src)%]
+
+Output:
+
+[%CFEngine_include_snippet(useringroup.cf, #\+begin_src\s+example_output\s*, .*end_src)%]