patch 8.2.0964: TextYankPost does not provide info about Visual selection

Problem:    TextYankPost does not provide info about Visual selection.
Solution:   Add the 'visual' key in v:event. (closes #6249)
diff --git a/runtime/doc/autocmd.txt b/runtime/doc/autocmd.txt
index 613dd85..5713119 100644
--- a/runtime/doc/autocmd.txt
+++ b/runtime/doc/autocmd.txt
@@ -1154,6 +1154,8 @@
 						register.
 				   regtype	Type of the register, see
 						|getregtype()|.
+				   visual	True if the operation is
+						performed on a |Visual| area.
 				Not triggered when |quote_| is used nor when
 				called recursively.
 				It is not allowed to change the buffer text,