Manual.rst: add internal link

Documentation/Manual.rst

@@ -2194,7 +2194,7 @@ General
 
     * Fields of the primary table acts like a *simple* form, all other fields have to be specified with *action/afterSave* records.
 
-  * *Multi* form: the form acts simultaneously on more than one record. All records use the same *FormElements*.
+  * *Multi* form: (multi-form_) the form acts simultaneously on more than one record. All records use the same *FormElements*.
 
     * The *FormElements* are defined as a regular *simple* / or *advanced* form, plus an SQL Query, which selects and
       iterates over all records. Those records will be loaded at the same time.