[Minor] Slightly improve the documentation

src/lua/lua_task.c

@@ -716,9 +716,11 @@ LUA_FUNCTION_DEF (task, get_dkim_results);
 /***
  * @method task:get_symbol(name, [shadow_result_name])
  * Searches for a symbol `name` in all metrics results and returns a list of tables
- * one per metric that describes the symbol inserted. Please note that this function
- * is intended to return values for **inserted** symbols, so if this symbol was not
- * inserted it won't be in the function's output. This method is useful for post-filters mainly.
+ * one per metric that describes the symbol inserted.
+ * Please note, that for using this function you need to ensure that the symbol
+ * being queried is already checked. This is guaranteed if there is a dependency
+ * between the caller symbol and the checked symbol (either virtual or real).
+ * Please check `rspamd_config:register_dependency` method for details.
  * The symbols are returned as the list of the following tables:
  *
  * - `metric` - name of metric
@@ -777,7 +779,11 @@ LUA_FUNCTION_DEF (task, process_ann_tokens);
 
 /***
  * @method task:has_symbol(name, [shadow_result_name])
- * Fast path to check if a specified symbol is in the task's results
+ * Fast path to check if a specified symbol is in the task's results.
+ * Please note, that for using this function you need to ensure that the symbol
+ * being queried is already checked. This is guaranteed if there is a dependency
+ * between the caller symbol and the checked symbol (either virtual or real).
+ * Please check `rspamd_config:register_dependency` method for details.
  * @param {string} name symbol's name
  * @return {boolean} `true` if symbol has been found
  */