docs(admin): Add note about configuring encoding order for text

Signed-off-by: Julius Härtl <jus@bitgrid.net>

admin_manual/configuration_server/text_configuration.rst

@@ -30,3 +30,13 @@ Rich text editing can be turned off globally to always open markdown files in th
 ::
 
  occ config:app:set text rich_editing_enabled --value=0
+
+
+ File encodings
+ --------------
+
+ Text can automatically detect encodings of files and will convert them to UTF-8 when saving. Due to the variety of encodings, not all of them can be detected, however you can configure a list of encodings and in which priority they should be detected using the php ``mbstring.detect_order`` setting in your php.ini:
+
+::
+
+    mbstring.detect_order         = ASCII,JIS,UTF-8,SJIS,EUC-JP