providers/saml: add custom help text for templates, add docs for User Object reference

docs/reference/property-mappings/user-object.md

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+# Passbook User Object
+
+The User object has the following attributes:
+
+ - `username`: User's Username
+ - `email` User's E-Mail
+ - `name` User's Display Name
+ - `is_staff` Boolean field if user is staff
+ - `is_active` Boolean field if user is active
+ - `date_joined` Date User joined/was created
+ - `password_change_date` Date Password was last changed
+ - `attributes` Dynamic Attributes
+
+## Examples
+
+List all the User's Group Names
+
+```jinja2
+[{% for group in user.groups.all() %}'{{ group.name }}',{% endfor %}]
+```