patch 7.4.786
Problem:    It is not possible for a plugin to adjust to a changed setting.
Solution:   Add the OptionSet autocommand event. (Christian Brabandt)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index b24a2a3..1a9e4f3 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -1535,6 +1535,15 @@
 		than String this will cause trouble.
 		{only when compiled with the |+viminfo| feature}
 
+						    *v:option_new*
+v:option_new    New value of the option. Valid while executing an |OptionSet|
+		autocommand.
+						    *v:option_old*
+v:option_old    Old value of the option. Valid while executing an |OptionSet|
+		autocommand.
+						    *v:option_type*
+v:option_type   Scope of the set command. Valid while executing an
+		|OptionSet| autocommand. Can be either "global" or "local"
 					*v:operator* *operator-variable*
 v:operator	The last operator given in Normal mode.  This is a single
 		character except for commands starting with <g> or <z>,