Added 3.1.2 changelog

docs/en/changelogs/3.1.2.md

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+# 3.1.2 (unreleased)
+
+## Upgrading
+
+### Treedropdownfield showsearch defaults to true
+The showSearch option of TreedropdownField is now set to true by default. This is to provide a fallback ui for when children of a tree node fail to render (due to too many children). You may set search as false when initializing a TreedropdownField, or afterwards:
+
+	:::php
+	$treedropdownfield->setShowSearch(false);
+
+If your data requires a specialized search function, you may specify it within:
+
+	:::php
+	$treedropdownfield->setSearchFunction();