MXS-2423: Add missing parameters to maxscale endpoint

Also updated the REST API documentation to include the newer output
(automating this update would be valuable).

include/maxscale/session.h

@@ -620,6 +620,11 @@ char* session_set_variable_value(MXS_SESSION* session,
  */
 void session_set_retain_last_statements(uint32_t n);
 
+/**
+ * Get retain_last_statements
+ */
+uint32_t session_get_retain_last_statements();
+
 /**
  * @brief Retain provided statement, if configured to do so.
  *
@@ -669,6 +674,11 @@ void session_set_dump_statements(session_dump_statements_t value);
  */
 session_dump_statements_t session_get_dump_statements();
 
+/**
+ * String version of session_get_dump_statements
+ */
+const char* session_get_dump_statements_str();
+
 /**
  * @brief Route the query again after a delay
  *