patch 8.2.2524: cannot change the characters displayed in the foldcolumn

Problem:    Cannot change the characters displayed in the foldcolumn.
Solution:   Add fields to 'fillchars'. (Yegappan Lakshmanan, Matthieu Coudron,
            closes #7860)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index d5264ec..7074608 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -3247,6 +3247,9 @@
 	  stlnc:c	' ' or '='	statusline of the non-current windows
 	  vert:c	'|'		vertical separators |:vsplit|
 	  fold:c	'-'		filling 'foldtext'
+	  foldopen:c	'-'		mark the beginning of a fold
+	  foldclose:c	'+'		show a closed fold
+	  foldsep:c	'|'		open fold middle character
 	  diff:c	'-'		deleted lines of the 'diff' option
 	  eob:c		'~'		empty lines below the end of a buffer