clarify plugin-load usage in tokudb.cnf file

storage/tokudb/tokudb.cnf

@@ -5,5 +5,11 @@
 # See https://mariadb.com/kb/en/tokudb-differences/ for differences
 # between TokuDB in MariaDB and TokuDB from http://www.tokutek.com/
 
+# Note, that only one plugin-load option will be recognized by the
+# MariaDB server. If you uncomment the line below, make sure that
+# you don't also have plugin-load options in your other
+# my.cnf files. Otherwise, add ha_tokudb.so to the existing
+# plugin-load option, instead of uncommenting the line below.
+
 #plugin-load=ha_tokudb.so