DOC: add config doc

src/doc/extension/edit_mode.md

+
+Customizing the Edit Mode
+=========================
+
+
+Customizing the number of candidates retrieved for references in the Edit Mode
+------------------------------------------------------------------------------
+
+To prevent the retrieval of huge numbers of entities just to create candidates
+for the drop down of a reference during editing of an Entity, the number of
+candidates can be limited.
+
+Set the ``BUILD_MAX_EDIT_MODE_DROPDOWN_OPTIONS`` build variable. -1 means no
+limit; every other number is the limit for each query sent (The number of
+candidates can be at most twice this number).